838° The American Naturalist. [October, 
The author applies Prof. Alpheus Hyatt’s “Classification of Stages — 
of Growth and Decline” to the brachiopods from the developing of 
the ovum to the old age of the individual. This classification works 
so well in this new application that it adds strength to it as a system. 
Prof. Beecher reviews the existing knowledge of the embryology of 
brachiopods, rendering very clearly the progressive development of 
the shell and associated parts. Kutorgina is suggested as a radical of — 
the strophomenoids. A close comparison is made of the reflected 
“collar” in developing Spirorbis, with the reflected mantle lobes in 
Cistella. Thecidium is considered as a surviving member of the stroph- 
omenoids, which group has previously been considered as extinct. 
Important observations are made on the development of the deltid- 
ium, in which the author shows that it is primarily a plate formed on 
the dorsal side of the posterior or pedicle segment of the larva. In 
later growth the deltidium becomes ankylosed with the ventral valve, 
which grows around so as to include it. This conclusion is strength- 
ened by collateral proofs of microscopical structure. Deltidial plates, 
on the other hand, he shows are developed by the unfolding of the 
ventral mouth lobes at the pedicle area. They therefore properly 
belong tothe ventral valve. 
A perforation in the umbo of the dorsal valve in many early articu- 
late types leads to the conclusion that they had an anus. In brachio- 
pods as a whole some features are progressive, others retrogressive. 
The protegulum’ or larval shell is mentioned, but is fully discussed in 
the earlier paper. 
Acceleration of development is clearly shown in Discinisca, which 
in the nepionic stage adopts characters which are first found in the 
nealogic stage of its ancestor, Orbiculoidea. Nice distinctions are 
made between characters acquired by inheritance and those adopted 
by special adaptations to conditions of environment, which latter may 
appear anywhere in separate genetic series. Postembryonic stages are 
briefly considered in types of the four orders proposed by the author. 
Old age, or the geratologic period in brachiopods, is marked by the 
thickening of the valves, and may be further ‘indicated by loss of = 
ornamentation and resorption of the deltidium or deltidial plates. m 
the early forms of each genus and family the species are small; in thè 
culmination they attain a maximum of size; before extinction they 
again resume a depauperate size and present abundant geratologous 
and pathologic forms. As such degraded types, Cistella and Gwynia, 
among brachiopods, bear such relations to the Terebratuloids as Bac- 
ulites amongst cephalopods do to the — 
BERT T. JACKSON. 
*Comparable to the protoconch and prodissoconch of mollusks. 
