1890.] Studies of Pelecypoda. 1139 
in origin to Pterinopecten, a Devonian genus which is adopted 
as the base of the family of the Pectinide. At a still later stage 
(Fig. 14), the hinge line is relatively shorter than in Fig. 13, yet 
lénger than in the adult. The ears are well pronounced. This 
stage is comparable to Aviculopecten (as amended by Prof. 
James Hall), which is the second step in the Pecten series. 
Similar stages to those described in Pecten irradians have been 
observed in several other species. In the young of Pecten ma- 
gellanicus there is ornamentation of the shell strikingly like that 
seen frequently in Aviculopecten. 
Hinnites is pecteniform in its nealogic stage, and shows 
graphically the change in form induced when it becomes attached 
and assumes the ostrean features characteristic of the adult. 
Spondylus has a prodissoconch like Pecten, and in its nepionic 
stage is pecteniform as observed in five species, proving its origin 
in that group. It early becomes attached and assumes the ostrean 
form. Plicatula becomes attached at the close of the prodisso- 
conch stage, and at once assumes the ostrean form; thus 
as in Ostrea nepionic characters are eradicated which might, 
if existent, show distinct phylogenetic stages. It is the furthest 
removed from Pecten of its series, because it earliest and most 
completely assumes the ostrean form. 
In Anomia the form of shell and peculiarities in the soft parts 
may fairly be ascribed to adaptation to the environment, rather 
than to genetic connections of the group. In the early nepionic 
stage Anomia is freely locomotive, with a highly active foot. At 
this age byssal attachment is voluntary, and is frequently aban- 
doned. The relations of the muscles, which are ambiguous in the 
adult, are clear in the young. The calcareous plug is composed 
of consolidated parallel ridges of lime. In the young these ridges 
are seen to be made up by the amalgamation of calcareous discoid 
points. The several points are directly comparable to the dis- 
coid points of fixation of the byssal threads in Pecten, Mytilus, 
etc. The prodissoconch of Anomia is described in two species. It 
differs from the prodissoconch of the previously mentioned 
genera in possessing a slight byssal sinus in one or both valves, 
according to the species, thus showing a very accelerated develop- 
