96 Annals of jhe Carnegie Museum. 



divided by a diagonal line, into anterior and posterior parts (Fig. 14). 



In the dorsal valve are two pairs of adductor scars. On each side 

 of a low rounded median septum is a small oval anterior adductor scar. 

 At the sides and a little posterior are the larger posterior pair. These 

 scars are surrounded by low ridges which are more strongly developed 

 at the back and sides than in front (Fig. 15). 



The median septum in this valve is produced for some distance in 

 front of the muscle scars and in the adult and senile specimens is 

 rather high and strong. Beginning just in front of the middle of the 



15 



Fig. 13. Pholidostrophia ioivaensis (Owen), ventral valve showing llie acumi- 

 nate cardinal extremities of young specimens. X 3 



Fig. 14. The same species. Interior of ventral valve, to show the adductor and 

 diductor ir.uscle scars. Natural size. 



Fig. 15. Interior of dorsal valve of the same species showing the adductor 

 muscles scars and the ridges connected with the brachial impressions. Natural size. 



Fig. 13. From a specimen in the Yale University Museum collection. Figs. 14 

 and 15 from specimens in the writer's collection. 



scars, on each side of the septum is a low ridge which curves gently 

 outward and then inward again, turning in rather abruptly as a sort of 

 hook at the anterior end (Fig. 15). 



In adults these processes extend about two thirds of the distance to 

 the front of the shell. In the young, they extend somewhat further 

 forward and are more divergent. These ridges correspond, in posi- 

 tion, with the brachial ridges of Choiietcs, Productus, etc., and should 

 probably be correlated with those markings.' 



The genus Pholidostrophia was suggested by Hall and Clarke to in- 

 clude a section of Stropiicodonta in which the shells were concavo-con- 



1 Ridges much like these, in Sfrop/teodonta kblanci Rouault were described by 

 (Khlert as belonging to the brachial apparatus. In describing the interior of the 

 dorsal valve of that species he says, " Les empreintes des muscles adducteures . . . 

 .sont tres rapproaches de la ligne cardinale et occupent de chaque cote du septum une 

 surface fortment bilobee ; entre chacun de ces lobes prennent naissance deux petits 

 bourrelets saillants, recourbes en hamegon, qui sont connus, dans la famille des Pro- 

 ductidse, sous le nom d'emprientes reniformes, et qui doivent appartenir a I'apparei- 

 brachial." Annales des Sciences, Geologic, Vol. XIX, p. 63, pi. IV, fig. 10. 



