1890.] Studies of Pelecypoda. 1141 
hinge line, the centralization of connecting tissue and teeth in the 
middle of the hinge line, and the development of umbos. 
The characters described in the prodissoconch of the Avicu- 
lide and their allies are found combined in a remarkable degree 
in the ancient genus Nucula, which, though living to-day, ex- 
tends back to early Paleozoic formations. It is probable that 
Nucula, or a nuculoid form, is the type we are seeking as the 
ancestral radical represented by the completed prodissoconch in 
the development of Avicula, Perna, Ostrea, Pecten, and their 
allies. The fact that Nucula is found in the Lower Silurian, and 
still lives without sufficient changes in form to make these ex- 
tremely separable forms in time generically separable, argues for 
a high antiquity for this genus. It was so firmly established in 
its earliest-known forms, it must at that time have been an ancient 
genus. 
The nepionic stages of Avicula, Perna, and Pecten (Figs. 8, 9, 
and 11) agree closely in form, and are referable to an early, sim- 
ple aviculoid type, Rhombopteria (Fig. 16). Therefore, that 
genus is adopted as the basis of the Aviculidz, being the first 
step up from the nuculoid radical of the 
group. From Rhombopteria, Leptodesma 
leads up to Avicula, forming the central stock 
of the family. From the Avicula stock many 
side issues arise, as Meleagrina, Pseudomono- 
| tis, Cassianella, Malleus, and others. These 
pu We CER are like Avicula (Fig. 9) in the nepionic stage, | 
onu uit. Baz, but in later growth depart from the typical 
Barrande). form of the stock. Perna is an important 
issue from Avicula, which it resembles in the nepionic stage. 
Gervillia and the Inoceramus group are modified forms from the 
Perna stock. The Ostreide are considered a modified branch 
from the Perna stock, which owes its peculiar features chiefly to 
the effects of fixation. Ostrea is the base of its group, and 
Exogyra the extreme, because it is the most highly modified by 
the adopted habit of attachment. 
7 A new genus, proposed in my paper for an ancient simple group of aviculoid shells. 
