PHYLOGENY OF THE PELECYPODA. 



329 



Fia. 30. — Young Periia ephipinmn, right valve; p, prodissoconch ; I, single triangular 

 cartilage pit ; c, cardinal and t, lateral teeth. X 90 diam. ' 



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

 cartilage pits and two lateral teeth. Lettering as iu flg. 30. X 24 diam. (Drawn by the 

 author.) 



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

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

 The hinge of this eai-ly i^eriod of growth, fig. 30, is markedly difierent 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 oi-iginate 

 on the hinge line each 

 being ti'iangular at first, 

 but not as acutely so as 

 is the first formed pit. 

 Hinge teeth exist in 

 young Perna (figs. 30- 

 31), although wanting 

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

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

 in either valve. The teeth corresjjond with similar teeth in a STpecimen of Avicula sterna 

 with which they were compared and they are found in specimens of Perna e-phi-pviam 

 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 jiits originate on the hinge line, and the triangular outline char- 

 acteristic 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 

 gi'owth, 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 period of Perna is probably closely like that de- 

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

 i.sts, 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. xxvii, figs. 2-3) 

 occupies an antero-ventral position in the pi'odissoconch shell, and a similar position was 

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

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

 considered as affording 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 dimyariau at that stage. The foot of 

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

 locomotion; it bears a deep longitudinal distal cleft as iu Pecten, and from a second 

 deeper proximal cleft, the byssus originates. The ^prodissoconch of Perua iu shell feat- 

 ures and implied anatomical parts is referable in origin to the Nuculoid radical, which 



