SYSTEMATIC PALAEONTOLOGY

Type species.- Umiaites rajnathi Spath, 1931
v. 1931. Umiaites rajnathi Spath, p.548-549, pl. 91, figs. 10 a-b; pl. 101, fig. 8
v. 1931. Umiaites minor Spath, p.549-550, pl. 92, figs. 1 a-b; pl. 102, fig. 6
v. 1994. Spiticeras cf. ducale (Matheron) Krishna, Pathak, Pandey, p. 333-334, pl.1, fig. 3

Material. The present study is based on six specimens including the types (Nos. 16213, 16214; re-illustrated here as Fig. 2.1 – 2.4 and Fig. 2.5 – 2.7) reposited at the Geological Survey of India, Kolkata. Two specimens are broken whorls of phragmocones. G. S. I. type no. 16213 is the holotype. Additional specimens have been collected from 2 km northeast of Lakhapar, western Kutch, and are kept in the museum of the Department of Geological Sciences, Jadavpur University.

Description. Shell is large and fully septate. Incomplete specimen (No. 3.1) diameter up to 104 mm; reconstructed diameter of the complete shell is about 160 mm (the presence of. another full revolution is evident from the trace of the umbilical seam of the body whorl (Figure 2.8; Figure 3.1-3.3). Shell is evolute (U/D = 0.39 to 0.56), inflated (W/H = 0.52 to 0.86), and shows wide intraspecific variability. Umbilicus is shallow and wide. Flanks are flat to gently curving; the radius of curvature increases ontogenetically. Whorl overlap is one-fourth of the preceding whorl height. Umbilical shoulder is rounded, and the wall is steeply inclined to overhanging, becoming more rounded on the outer flank and gradually merging with narrowly arched venter. Shell is marked by a deep adorally concave constriction on the flank, conformable with the primary ribs, but secondaries abut against the posterior end of the constriction. Innermost whorls are obscured by matrix. Primary ribs are seen at 25 mm diameter initially. They appear to be rectiradiate, fine and closely spaced. First constriction is seen at about 30 m diameter. At about 40 mm diameter both primary ribs and constrictions show a rursiradiate pattern. Primaries become increasingly strong and separated. At least two constrictions are present per whorl, and the last constriction is noticed at a phragmocone diameter 100 mm. Whorl section is elliptical with maximum width lying just below the mid-flank. Primary ribs are strong, coarse and distant. They originate from the umbilical wall and rise slightly rursiradiately up to the umbilical margin and then flex forward. Strength of primary ribs decreases posteriorly and below the mid-flank they almost disappear (on internal mold) and a bundle of secondary ribs arises with variable abundance during growth. At 70 mm diameter, three secondary ribs occur with irregularly placed intercalatory ribs. Secondary ribs are fine, closely spaced and sharply crested. Late phragmocone whorl may include six secondary ribs. Upon initiation, secondary ribs bend forward and cross the venter with much adoral projection. This projection is increasingly pronounced during ontogeny. The number of primary and secondary ribs is 10 and 60, respectively, including intercalatory ribs, per half-whorl at 104 mm diameter. Number of primary ribs appears to be more or less uniform in different ontogenetic stages. They become somewhat broad and flat, and secondary ribs become progressively prominent and sharply crested toward the preserved end. This stage may represent an adult phragmocone.

Septal suture moderately complex with trifid, deeply incised, narrow first lateral lobe; first lateral saddle short and relatively wide; the auxiliaries are also equally frilled and deeply indented (see Spath, 1931, pl.101, fig. 8; pl. 102, fig. 6).

Discussion. Spath (1931) first described two species of Umiaites as U. rajnathi and U. minor. Both species have been described on the basis of septate specimens, and U. minor is based on a monotypic holotype, which is an immature shell. Spath (1931) is known for his excessive subjective splitting which led to an abundance of species names (see e.g., Cariou and Krishna 1981 and Jana et al. 2005). In the present case, Spath (1931) noticed that the inner whorls of U. rajnathi closely resembles U. minor in nature of ornamentation. Both are equally evolute (U/D is 0.41 and 0.42, respectively, for U. rajnathi and U. minor). U. minor, however, is less compressed than U. rajnathi (W/H is 0.86 and 0.58, respectively). Our additional specimens reveal that Umiaites population shows wide intraspecific variability with respect to degree of inflation. The type specimens of both U. rajnathi and U. minor represent two extreme ends and all additional topotypes fall intermediately within the spectrum (Table 1). Therefore, it appears Umiaites population in Kutch is a homogenous assemblage which shows continuous variation, a character of a single biological species. U. rajnathi and U. minor therefore are considered conspecific.

Umiaites shows stunning similarities with Proniceras in many morphological aspects. They strongly resemble each other in degree of involution (Figure 4.1) and degree of inflation in early whorls (Figure 4.2). They also correspond well in ribbing pattern. The secondary ribs form typical forward projected sinuses, which were described by Spath (1931). Moreover both have strong primary ribs and finer secondary ribs, which are forwardly projected especially on the venter as noted in both versions of Treatise (Arkell et al. 1957; Wright et al. 1996). However, Arkell et al. (1957) described the outer whorl of Umiaites, based on the holotype, as having a smooth shell. This description might prompt one to falsely conclude that Umiaites has a smooth body chamber. Spath's (1931) illustrated example of the holotype (pl. 41, fig. 10 a, b; here refigured in Figure 4a) is still septate and bears both primary and secondary ribs. However, Proniceras is a small genus (the maximum adult size is about 77 mm) and is a lappeted microconch. It has more numerous primary ribs (14 to 17) per half whorl, which are also sharper and stronger. Secondary ribs in Proniceras are also relatively few (two to three). Thus, Proniceras corresponds well with the early whorls of Umiaites.

Krishna et al. (1994) described Spiticeras cf. ducale (Matheron) from our collecting horizon and locality of the Umiaites in Kutch. Spiticeras is compressed, with ribs furcating high up on the sides into numerous secondary ribs, and lacking tubercles. It most closely resembles Umiaites and is dissimilar to Spiticeras Uhlig, 1903, the latter being coronate with the furcation point lying near the umbilical margin. Recently Yin and Enay (2004) correctly pointed out that the solitary specimen of Krishna et al. (1994) does not belong to Spiticeras, assigning it instead to Proniceras. However, the specimen is septate, has a diameter of 73 mm (incomplete specimen) and has multiple forwardly projected secondary ribs. Thus, it strongly resembles the present Umiaites rajnathi and is synonymised here. The tentative designation by Yin and Enay (2004) illustrates the fact that Proniceras is poorly distinguishable from the inner whorls of Umiaites, and that Umiaites was inadequately known to other workers mainly because of lack of illustration and incorrect description in the early Treatise (Arkell et al. 1957).