BRAOHIOPODA. 913 
The purely conventional value of family designations could not 
be more forcibly illustrated than among the groups now under 
consideration, and as shown by the following facts: Among the 
Oz. ,t1p# there is, in a certain direction, a tendency manifested 
toward the formation of a thickened muscular, platform, a struc- 
ture to which we have already referred as being most highly 
developed in the trimerellids (Trimmretta, Divozoxts, etc.). This 
is seen in the Cambrian genus Exxania, where these platforms 
are solid muscular thickenings of the shell, and their development 
is more highly advanced in Laxumrna, also of Cambrian age, in 
which the platforms are vaulted or excavated beneath. Dryoso- 
Lus, Which makes its appearance in the early faunas of the 
Silurian (Black River limestone), is a large shell of oboloid 
exterior and having these platforms well developed, sometimes 
solid, but often excavated. This genus continues its existence or 
rather, in accordance with our knowledge, reappears after a long 
interval, in the later faunas of the Upper Silurian in a fuller 
manifestation and in association with other platform-bearing 
genera, TrimeRELLA, Monomuretia, Dinosowus. 
Among the Lineviip# a similar tendency to the formation of 
these platforms manifests itself, though at a later period than 
in the oboloids. The elementary condition of development 
is expressed in LineuLors, and in a more advanced stage in 
Linevrasma, of the middle Silurian. In Trmerziia, Mono- 
MERELLA and Ruinozorus of the later Silurian the lingu- 
loid form of the shell is retained and the highest stage 
of platform development attained. These shells have been 
closely studied ; our knowledge of them is fairly complete, suffi- 
cient to justify us in the conclusion that in the TrmereLLipz 
(of Davipson and Kine; a very natural group, including 
TrimereLia, Monomereiia, Rarosotus and Drnozotvs, to which 
we should add Linetrors and Liveutasma), the derivation of the 
platform has been along two distinct and convergent phyletic 
lines, one departing from the oboloids, the other from the 
linguloids. The line of the platform bearing Inarticulates ends 
abruptly and finally with the close of the Silurian. The tend- 
ency to produce these platforms, which was then common to the 
oboloids and linguloids, and hence a heritage from common 
ancestry fails to manifest itself in other lines of departure 
115 165 ; 
