180 CKWTACUOUS LAMtiLLI BltANCHIA. 



Measurements .• 













(1) (2) 



(3) 



Length 



. 



80 . 70 



69 mm. 



Height 





44 . 39 



ob „ 





(1- 



-3), Perna-bed, Atherfield. 





. Infinities. — Forbes states that English examples of this species were identified 

 by comparison with French specimens. I have not seen any foreign examples of 

 P. Bobinaldina, but the English specimens appear to differ from those figured by 

 d'Orbigny and by Pictet and Campiche in their greater convexity and larger size, 

 in the more acute angle which the ribs make at the dorso-ventral line, and in the 

 somewhat less anterior position of that line. In all these respects the English 

 examples approach more nearly to P. neocomiensis (de Loriol), 1 but differ from that 

 species in their more elongate form. It seems, however, not unlikely that a large 

 number of specimens might show that P. Bobinaldina and P. veocomiensis are not 

 specifically distinct. In connection with this, attention may be called to Nucula 

 (Acila) bivirgata, Sowerby, in which the ornamentation is somewhat similar and 

 shows considerable variation. 



The genus Ptychomya was placed in the Crassatellitidai by Pictet and Campiche 

 and by Stoliczka. Its resemblance to Circe (Crista) was pointed out and fully 

 discussed by Dames, 2 whose view of its systematic position is accepted by Fischer. 



Remarks. — The date of the plate on which Agassiz figured Ptychomya plana is 

 earlier, but the text relating to it is later than d'Orbigny's figure and description 

 of Crassatella Bobinaldina. Since the specimen figured by Agassiz is rather 

 unsatisfactory and d'Orbigny's name has been used by nearly all writers it seems 

 desirable to retain the latter. 



Distribution. — Lower Greensand (Peraa-bed) of Atherfield. Ferruginous 

 Sands of Shanklin. Hythe Beds of Hythe, Lympne, and Maidstone. 



1 Pictet and Campiche, "Terr. Oct. Ste. Croix" ('Mater. Pal. Suisse,' ser. 4, 1866), p. 355, 

 pi. exxvii, figs. 9 — 12. 



2 ' Zeitsclir. d. deutsch. geol. G-esellsch.,' vol. xxv (1873), p. 374, pi. xii, figs. 1 — 4. 



