362 EGBERT TRACY JACKSON ON THE 



socoiich of A. aculeata has a groAvth similar to that described in A. glabra,' but is chalky 

 Avhite and much thicker than in that sjjecies. The I'iglit valve is wholly or for the most 

 part composed of pi'ismatic cellular structure whereas that tissue is wanting in the left 

 valve as in A. glabra. My observations of the young and adult lead me fully to concur 

 with Professor Yerrill Avho says that he believes this species is perfectly distinct from 

 Ajiomia glabra. 



In regard to the affinities of Anomia, it is closely related to Placuna and many inter- 

 mediate forms may be traced as noted by authors. The anatomy of Placuna is essentially 

 like that of Anomia; but the mantle border has a curtain like Pecten (Tryon), The 

 shell of Placuna is composed of fine nacreous layers, but in the young I have found 

 traces of prismatic cellular tissue in the right valve. This tissue was very thin and 

 limited in extent on account of being ^vorn away, but existed in patches. Woodward (79) 

 mentions that, when young, Placuna has a byssal sinus in the right valve and I have found 

 yoiing specimens which show an enclosed bj'ssal ])lug like that of Anomia, as described 

 in section xvi. Anomia, anatomically and in shell structure, is much more nearly allied 

 to Pecten than to Ostrea. It is probable that Anomia was derived from the Pectinidse 

 from the Amusium or Hemipecten group, for the latter resemble Anomia in having thin 

 nacreous shells. In considering Hemipecten, Zittel remarks that Jurassic species of Pla- 

 cunopsis are probably confounded with Hemipecten, thus inferring a close similai'it}' in 

 the tAvo genei'a. Placuna was probably derived from the early Anomiadas as all evi- 

 dence points toward an ancestry in that grouji. The geological appearance of these 

 groups accord with their inferred zoological affinities as I have described them. That 

 such a highly modified form of shell as Anomia could have been derived from the Pec- 

 tens is more easily believed when we see how highly modified are Pedum, Hinnites and 

 Spondylus which are more direct derivatives of Pecten. 



XIV. Studies of a few other Geneka. 



The genera so far studied are all linked in one great group of the Aviculidic and their 

 allies (see table, section xvi), by characters of anatomy and shell structure which connect 

 the several members. The genera thus referred to are Ostrea, Gryphsea, Exogyra, Perna, 

 Avicula, Monotis, Pseudomonotis, Cassianella, Vulsella, Pecten, Spondjdus, Plicatula, 

 Lima, Anomia and Placuna. I now wish to consider the j^oung- of some genera belong- 

 ing to other groujis of Pelecypoda distinctly removed from those previously considered 

 and not always closely connected with one another. Figures of the adults of most 

 of the species described in this section are to be found in the works of Verrill and 

 Gould. 



The groups, previously studied, have been characterized by possessing prodissoconcJis 

 of homogeneous laminar structure, but not jirismatic, and roitJi umbos directed posteri- 

 orly. The succeeding dissoconchs have been shown to be markedly different in form 

 from the prodissoconch, and in the earliest stages at least are characterized by ivell-de- 

 Jined, prismatic, cellular structure, either in both valves or the right valve.^ In all gen- 



' The change from homogeneous to prismatic structure do show it, that the absence of the feature in the few 



■was not observed in the fossils descriljed or iu Spondylus specimens observed need not be considered in this connec- 



and Lima, but these genera are so united with groups that tion. 



