OP SOUTHEEN INDIA. 445 



spinose ribs interposed ; both are often interrupted in tlieir course by concentric 

 lines or furrows indicating stages of growth. 



Localities, — Near Shillagoody, in a grey, conglomeratic soft sandstone ; 01a- 

 paudy, in brownish, finely oolitic rock ; rare. 



Formation, — Arrialoor group. 



2. Plicattjla instabilis, StoliczJca, PL XXXIV, Pigs. 3-14 and 19 ; PL XLVI, 



Pig. 3. 



PL testa ovata, paulo ohliqua^ apice valde august at a, incequivalvi, umhonibus 

 depressis, approxvmatis^ valva super a planata, infer a plus minusve convexiuscula, 

 nonnunquam accumulate lamellosa, ad umhonem hreviter affixa ; utraque radiatim 

 costata^ costis aut paucis et fortihuSy aut numerosis et tenuihus^ semper squamose 

 spinulosis, spinulis depressis, angustis ; dentihus cardinis in utraque valva ah apice 

 remotis, costiformibus, in valva supera remotis, costula ligamentali medio separatism 

 in valva infera magis approximatis, duplicibus, partibus internis valde prominentibus, 

 externis marginalibus, nonnunquam sub-obsoletis ; impressione musculari ovato rotun- 

 data, excentrica ; impressione palliali profunde impressa, continua, ab margine remote 

 sita. 



This species resembles in its ovate shape the last, but the lower valve is always 

 somewhat convex, while the upper is flat, or very nearly so. The ornamentation 

 is extremely variable, individually and from cau.ses of preservation of the surface. 

 It is almost difficult to find two specimens which would entirely agree in this 

 respect. As a rule, young specimens have from seven to nine ribs, but the 

 number rapidly increases by bifurcation. In very few cases, (see fig. 4, pi. xxxiv), 

 I have observed only four ribs ; immediately, however, the next stage of growth 

 begins, the number is doubled. In well preserved younger shells the ribs are 

 ornamented with more or less projecting, scaly spines, (see figs. 12a and 14 on 

 pi. xxxiv), and only when these are partially or wholly worn off, the ribs become 

 better discernible ; they never have any distinct thinner intermediate ribs, or striae, 

 between them. In the progress of growth, the ribs, as already stated, divide into 

 two or more branches, and when they were originally numerous (as seen in 

 fig. 11 on pi. xxxiv), the full grown shells offer such a densely ribbed and scaly 

 surface as is represented in figs. 19 and 19 a on the same plate. 



The internal structure of the valves does not offer any peculiarities ; in both 

 valves the hinge-teeth are somewhat removed from the umbones, and in the upper 

 or right valve there is a low median ridge to which the ligament was evidently 

 attached. This ligamental ridge is also indicated in several other tertiary and 

 recent species. 



Localities. — Chokanadapooram and Ootacaud, in a white earthy limestone, 

 very common and mostly of larger size ; west and south-east of Arrialoor, in grey 

 sandstone, common, but all specimens are of small size; other localities are 

 Olapaudy, ComarapoUiam, Shillagoody, and north of PoodoopoUiam, in silicious 

 or calcareous sandstone. 



Formation,- — Arrialoor group. 



5 R 



