

390 JACKSON ON THE PELECYPODA. 



shell is induced by the conditions of fixation in any attached Pelecypod; therefore Dimya 

 should be considered as a highly modified branch from Pecten, which is ostreaform be- 

 cause attached. It is shown in this paper that the prodissoconch of Ostrea, Pecten, etc., 

 is dimyarian, therefore we may fiiirly infer that the existence of two adductor muscles in 

 adult Dimya is due to the retention of both muscles which are doubtless existent in the j^ro- 

 dissoconch stage of that genus. It is to be noted also that the gills of Dimya are in an 

 archaic condition of simplicity as described by Dall. It is shown in the table of genera, 

 p. 391, that Pecten and Ostrea are very widely separated genera, therefore no one genus 

 can propei'ly be considered as intermediate between those two types. Dimya is consid- 

 ered as a side-issue from Pecten, jjossessing many archaic and modified features which 

 render it a highly retrogressive or degradational form. (Further description of Dimya, 

 see pp. 306, 324.) 



Pedum is evidently a descendant of Pecten. In the nepionic period, Pedum is purely 

 pecteniform in both valves, not yet having acquired the peculiai-ities of the genus. 

 Later, the byssal sinus is enclosed so as to become, in a measure, foraminal, and the shell 

 is apparently closely related to the object of byssal attachment. The right valve is the most 

 highly modified, although both valves are modified, and in the adult condition bear little 

 resemblance to Pecten. The peculiar form of the shell of Pedum, it may be reasonably 

 inferred, is due to the habit of close byssal fixation. 



Hinnites, as already discussed, is an important genus in our conclusions. Up to the 

 nealogic period, it is free or attached by a byssus and pecteniform, PL xxvi, figs. 3-4. 

 It then becomes permanently attached by soldering the i-ight valve to an object of sup- 

 port and with this change in habit we find that the irregular ostreaform growth is 

 introduced as fully discussed (section vii). Hinnites is an undoubted descendant fiom 

 Pecten and owes the peculiarity of its form to its adojjted habit of fixation. (Further 

 description of Hinnites, see pp. 324, 350.) 



Spondylus, like Hinnites in its nepionic stage, is pecteniform, PI. xxvii, fig. 4; then 

 becoming attached at this eai-ly stage, the irregular growth characteristic of the genus 

 is introduced and the shell becomes spondyliform. The right attached valve is the most 

 highly modified, and the left valve is the least modified and nearest approaches to Pecten 

 in form (discussed in sections vii and xi). Spondylus is considered as descended from 

 Hinnites and is more highly modified than that genus because it earlier adopts the con- 

 dition of fixation. The degree to which a talon is developed in Spondylus varies, being 

 least developed in species which are most nearly equivalvular (p. 352). The peculiar in- 

 terlocking teeth of Spondylus are believed to be a modification of the auricular crurae 

 characteristic of most Pectens and especially Amusium. The several teeth of Spondylus 

 in the young seem dii-ectly traceable to the knol:»-like processes of the crura} existing at 

 the auricular termination of the crui-^e in Pecten. In Spondylus apparently by the short- 

 ening of the crm'fe, the distal knobs of the same are brought in close proximit}^ to the 

 hinge Hue, where they are further modified so as to produce interlocking, articulating 

 processes. If this assumption in regard to the teeth of Spondylus is correct, they are 

 teeth of different morphological nature from ordinary cardinal teeth and ai"e to be com- 

 pared to those only as to mechanical equivalents. (Further consideration of Spondylus, 

 see pp. 318, 323 and 351.) 



