TRANSACTIONS OF WAGNER 

 I I 34 



TERTIARY FAUNA OF FLORIDA 



rounded, the posterior longer and olDliquely pointed ; surface minutely granose 

 and with concentric incremental sculpture, a rounded ridge extending from 

 the umbo to the posterior basal angle ; beaks low, rather incurved ; hinge 

 delicate, a small subconical cardinal under the umbo in the right valve and a 

 slightly excavated nymph behind it for the reception of the ligament and 

 resilium, which are external ; base and posterior hinge-line nearly parallel, the 

 latter terminating at an angle of the dorsal margin beyond which the margin 

 slopes downward ; the outer edges of the nymphs are incised, forming a 

 narrow groove; interior of shell smooth, anterior muscular scar impressed, 

 posterior and pallia! line faint and obscure. Lon. 6, alt. 3.7, diam. 2.9 mm. 



A single right valve was obtained. From the Miocene form of the last 

 species this is distinguished by its greater length and more conspicuous granu- 

 lation. In reality, however, the differences between Fulcrella and the typical 

 Anisodonta are insignificant. 



Genus HINDSIELLA .Stoliczka. 

 Hindsia Deshayes, An. s. Vert. bas. Paris, i., p. 693, 1858 ; not of Adams, 1853. 

 Hindsiella StoL, Cret. Fauna India, Pelecypoda, p. 266, 1871 (type Modiola arcuata 



Def ranee, Desh., op. cit., p. 695, pi. 53, figs. 3^-35. 1858) ; Cossmann, Cat. 111., ii., 



p. S3. 1887. 

 Vasconia Fischer, Les fonds de la Mer, ii., p. 83, 1873. 

 Kellia (sp.) De Gregorio, Cossmann. 



The peculiar form of the shell with its median constriction, which led to 

 the institution of a genus for the Parisian fossil, is probably due to the com- 

 mensal habit, and may therefore occur in commensal species of different 

 genetic relations. My attention was first drawn to this explanation by the 

 discovery of the situs of " Pythiiia" rngifera Cpr. of the Alaskan fauna. This 

 little bivalve is byssiferous and has the same median arcuation as the Hind- 

 siella. It lives attached by the byssus to one of the abdominal segments of 

 Gebia pugetensis Stm., a burrowing crustacean of the northwest coast. The 

 mollusk, by means of its arcuate medial sinus, precisely fits the convex surface, 

 to which it is attached by its byssus, and it is difficult, after examining one 

 in situ, to doubt that its location and form have not a certain relation of cause 

 and effect. By fitting closely and thus being able to keep itself hanging sym- 

 metrically on the abdomen of the crab the mollusk avoids the shocks which 

 it would receive if it swung to one side, and is able to maintain its position 

 sheltered from the carnivorous gastropods always so ready to drill holes in 

 thin-shelled bivalves. Besides this the fragments of the crab's food in the 



