446 CEETACEOUS PELECYPODA 



3. Plicatula mijlticostata, Forbes, PI. XXXIV, Pigs, 15, 16, 17, and 18 ; 



PL XLVI, Pigs. 5-6. 



1846. Plicatula multicostata, Forbes, Trans., Geol. Soc, Lond., vii, p. 155, pi. xviii, fig. 3 ; eadem auctorum. 



JPL testa late ovata, rare sub-orbiculata^ apice rotundate ohtusata^ margine 

 posteriore convexo^ anteriore truncato aut paululum concavo^ incequivalvi ; valvis 

 convexiusGulis, indistincte auriculatis, inferiore altera paulo majored ad umhonem 

 hremter adnata^ rare late sessili^ utraque dense radiatim costulata^ costulis sub' 

 cequalibus^ in junioribus squamose spinulosis^ in adultis multiplicate divisis^ plus 

 minusve large spinulosis, in valva inferiore paululum distantioribus^ i/nterstitiis con^ 

 centrice squamiforme striatis. 



Shell broadly ovate, with obtuse slightly prominent beaks and indistinct ears. 

 Some specimens are a little more oblong than others, but the form very rarely 

 approaches to orbicular. The upper valve is a little smaller than the lower, which is 

 generally only with a small portion of the umbo attached ; rarely is the place of 

 attachment enlarged, and only in such case the form of the shell is subject to varia- 

 tion (see pi. xlvi, fig. 6). Both valves are convex, the lower slightly more so than 

 the upper. The surface is covered with numerous, very closely set, sub-equal radiat- 

 ing ribs, ornamented with scaly spines. As the shell increases in size, the ribs 

 multiply by division, and some of them often become stronger than others. The 

 scaly spines increase in proportion, and in well preserved shells considerably project 

 beyond the margin. The ribs are, as a rule, a little more distant on the lower than 

 on the upper valve (compare figs. 16 and 165 on pi. xxxiv). 



This species is very closely allied to PL aspera, Sow., from the Alpine Gosau 

 deposits, (see Zittel in Denksch. Akad., Wien, vol.xxv, pi. ii, p. 120). There does 

 not appear to be any essential distinction in the form and ornamentation of the 

 shells of the two species, but in aspera the upper valve is always flat or even 

 concave, while in multicostata the upper valve is always convex, near the beaks 

 sometimes even considerably tumid. 



The species described by d'Orbigny as PL aspera can hardly be regarded as 

 identical with the one from the Gosau ; good materials must be compared in order 

 to establish the relation of these two forms. We possess in our collection a specimen 

 from Tours, in which the radiating ribs are as distant as shown in d'Orbigny's 

 figure and the spinulose scales are of a different character. The American PL 

 urticosa, Mort., which Zittel identifies with aspera, is to all appearance a much 

 more elongated shell. 



Localities.— l^^^Y Serdamungalum, Alundanapooram, and Anapaudy ; common, 

 in a yellowish brown or bluish grey sandstone. 



Formation. — Trichinopoly group. 



4. Plicatula sessilis, Stoliczha, PL XXXIV, Pig. 21 ; PL XLVII, Figs. 5-7. 



PL testa oblique ovata, incequivalvi, valva infera convexa, ad umbonem truncata, 

 anguste vel latiuscule affixa, parte libera radiatim costulata, costulis cequalibus, squa^ 



