PHYLOGENY OF THE PELECYPODA. 



329 



Fig. 30. — Youna; Perna ephippimn, right valve; p, prodissoconch; I, single triangular 

 cartilage pit ; c, cardinal and t, lateral teeth. X 00 diam. 



Fig. 31. — Perna ephippimn, older .specimen than fig. 30, .showing several triangular 

 cartilage pits and two lateral teeth. Lettering as in fig. 30. X 24 diam. (Drawn ijy the 

 author.) 



Viewed from the left side, PI. xxvii, fig. 1, it possesses a rounded wing anteriorly and 

 posteriorly the wing is rounded but not produced beyond the borders of the valves. 

 The hinge of this early period of growth, fig. 30, is markedly different from the adult. 

 The cartilage pit is single, acutely triangular in outline, situated obliquely (as in Mal- 

 leus, PI. XXVI, fig. 19), 

 and extends downward 

 from the hinge area of 

 the prodissoconch valve 

 as in Ostrea, PI. xxiv, 

 fig. 19. As the shell 

 grows, fig. 31, new 



cartilage pits originate 



on the hinge line each 



being triangular at first, 



but not as acutely so as 



is the fii'st formed pit. 



Hinge teeth exist in 



young Perna (figs. 30- 



31), although wanting 



in the adidt, PI. xxvi, fig. 16. Two cardinal teeth in the right valve and one in the left 



are situated directly beneath the umbos. A slight posterior lateral tooth exists as well 



in either valve. The teeth corrcsjiond with similar teeth in a specimen of -4 yicwZa sterna 



with which they were compared and they are found in specimens of Perna e2)Mppium 



up to a centimeter or more in height. These teeth are apparently directly inherited 



from Avicula, which is in all probability the immediate ancestor of Perna. During later 



growth new cartilage pits originate on the hinge line, and the triangular outline char- 



acteiistic of the developing pits is lost, giving place to the perpendicular form of pits 



characteristic of the adult, PI. xxvi, fig. 16. The hinge teeth also disappear during 



growth, and the form of shell characteristic of the species is rapidly built up. 



The anatomy of adult Perna resembles that of Ostrea as already considered, section v, 

 and the anatomy of the prodissoconch jjeriod of Pei'na is probably closely like that de- 

 scribed in the oyster, p. 300, PI. xxiv, figs. 1-2. At least one difference, however, ex- 

 ists, and that is the presence of a well-developed foot. The foot of Perna as indicated 

 by the position of the notch in the succeeding dissoconch growth (PI. xxvir, figs. 2-3) 

 occupies an antcro-ventral position in the prodissoconch shell, and a similar position was 

 occupied by the foot in the young prodissoconch period of Avicula, fig. 33, p. 330, Pecten, 

 PI. XXVII, figs. 8-9, and Anomia, PI. xxix, figs. 6-7. The position of the foot may be 

 considered as afibrding evidence concerning the anatomy of the prodissoconch stage, as 

 it doubtless closely underlies the anterior adductor muscle, its normal position in dimya- 

 rian Pelecypods, for Perna was unquestionably dimyarian at that stage. Tiie foot of 

 adult Perna is small compared with that found in Pelecypods where it is used actively in 

 locomotion; it Ijears a deep longitudinal distal cleft as in Pecten, and from a second 

 deeper proximal cleft, the byssus originates. The prodissoconch of Perna in shell feat- 

 ures and implied anatomical pai'ts is referable in origin to the Nuculoid radical, which 



