JUKES-BEOWNE : ON TIVELA AND GRATELOUPIA. 273 



Further, I see no reason for retaining the PachydeHma of Conrad to 

 include a few species which liave a more vernicose periostracum than 

 the rest, for the supposed existence of four cardinal teetli in these 

 species is a mistake of Conrad's, repeated by Dali. The hinge of 

 Pachydesma (i.e. T. stultorum) has been noticed on p. 267, and its 

 dentition is merely one phase, among several, in the development of 

 the nymphal ridges. It would be just as reasonable to separate those 

 species which luive two such ridges as to make a section out of those 

 which have only one. 



The only form, I'ecent or fossil, which seems to liave a character of 

 siibgenerio importance is T. perplexa (Stearns), in whicli the margins 

 of the valves are crenulated, and for which Dall proposed the name 

 oi. Eutivela in 1891.' I liave referred to his description, which is 

 accompanied by a woodcut of the left valve, and there can be no 

 doubt that the shell is a Tivela. Judging from the figure the 

 crenulations of the margin are distinct, though not in deep relief. 



' The Nautilus, vol. v, p. 26. 



