230 The American Naturalist. [December 
Perna has a prodissoconch closely like that of Ostrea. The 
byssal sinus originates in the initial stages of dissoconch growth. 
In young Perna (Fig. 6) a single triangular cartilage pit exists as 
in Ostrea. In later growth new pits originate on the ligamental 
line (Fig. 7). They, too, are triangular at first, but when the 
sides of the triangles have attained a certain degree of divergence 
they are produced perpendicularly, which is the condition found 
in the adult. The nepionic stage of Perna (Fig. 8) is sub- 
rhombic in form, and the anterior auricle descends directly from 
the limits of the ren The form of this stage is com- 
parable to that found in early Paleo- 
zoic Aviculoids. It is compared to 
Rhombopteria glabra sp. Barr., from 
the Silurian. 
In Avicula we find a prodissoconch 
like that of Perna and Ostrea, and, as 
in those genera, it is traceable in 
origin to the nuculoid radical which 
forms the basis of our genealogical 
table. The nepionic stage of Avicu- 
la (Fig..9) is subrhombic in form, 
and is referable in origin to early 
Fides aae 7, rodisoc erna «Phi? Aviculoids. It is compared to Rhom- 
about 37 diam bopteria mira (Fig. 16), from the Silu- 
rian. A later stage of Avicula (Fig. 10) differs from the nepionic 
and adult periods, and is comparable to the Devonian genus Lep- 
todesma, which is the third step up in the phylogeny of the 
Avicula series. 
A prodissoconch and nepionic stage similar to those of Avicula 
are described in many allied genera, which are Meleagrina, Mono- 
tis, Pseudomonotis, Oxytoma, Cassianella, and Vulsella. 
_ Pecten irradians presents an interesting subject for study. Its 
life-habits are varied and striking. Its stages in growth, both in 
the soft parts and the hard, present a remarkable series of high 
phylogenetic significance. Atthe completed prodissoconch stage 
Pecten is doubtless dimyarian, and crawls on the ventral border 
of the valves by means of an active foot. It may also have a 
