1890.] Studies of Pelecypoda. 1135 
by cemented fixation is a concave attached valve, a flatter and 
commonly thinner free valve, an irregularity and assymmetry 
of growth tending to the displacement of characters normally 
found in near allies, and, as a general thing, a camerated 
structure of the shell. The fullest modification in this line 
of variation is the production of a shell in which the attached 
valve is cup-shaped, conical, or subcylindrical, as seen in 
the Chamidz and Rudiste. In this group as a whole, and in 
the progressive stages of growth of its extremest members, all 
transitions may be followed between a simple ostreiform or exogyri- 
form shell and the most highly modified conical type. The 
Ostrea form is the first step in this line of variation, the Exogyra 
form is the second step, and the Hippurite form is the last step, 
The equal impact of moving water on all sides of the attached 
organism is believed to be the chief factor in inducing this form, 
but aided by correlated forces as briefly discussed in my paper. 
Fic. 6.—Young Perna are right valve ; 7, prodissoconch ; Z, cartilage pit; c, 
cardinal, and 7, lateral, teeth. X go diam. ; : 
FIG. 7.—Perna ephippium, older than Fig. 6, with several cartilage pits ; lettering as 
above. X 24 diam. 
It is considered that Ostrea probably descended from Perna, or 
a close common ancestor of the two genera. The bases of like- 
ness are the anatomy of the soft parts, shell structure, the prodis- 
-soconch, and other features in the development of both genera. 
The differences are mainly ascribed to the effects of the condition 
of fixation in Ostrea. 
