914 Report oF THE STATE GEOLOGIST. 
Some of the accessory causes contributing to the production of 
the vaulted plates seem to have been efficient at various times 
throughout the history of the entire class, and we find them 
manifested in the Articulate Brachiopods by the marginal eleva- 
tion of the muscglar area in Dovviriina and Leprmsya; they 
probably also served to render more complete the isolation and 
perfection of the spondylium, but the true platform does not 
again appear. (For suggestions as to the organic causes of 
such elevations see p. 194.) With the knowledge of so many 
transition genera between the oboloids and linguloids and 
trimerellids, it is a pure convention to accord recognition to the 
families Osotipm, LineuLips and TRIMERELLID®. 
This entire group of genera is characterized by the presence of 
an unenclosed marginal pedicle. They compose the Mxsocavria- 
or Lineutaora of Waacen (Atremara of Bercuer). 
The second main division of the Inarticulate genera is com- 
posed of those in which the pedicle-aperture, in immature stages 
or in primitive adult conditions takes the form of a marginal 
incision of the pedicle-valve, but becomes enclosed in the shell- 
substance in later stages of growth. To this group Waagen 
applied the term Dracavur1a* or Discrnacea (1883). 
In the genus Orsioutorpsa, early stages of the shell have the 
pedicle-passage or open triangular incision extending with widen- 
ing margins from the apex of the pedicle-valve to the periphery. 
With advance of age this passage becomes contracted and 
normally closed at the posterior margin, and may be variously 
modified by the deposition of adventitious deposits about it. In 
the Cambrian genus Discryoxzris the open slit is a mature char- 
acter, and it also appears in several similar genera of later date, 
sometimes slightly modified, e. g., Trematis, Scmzoxoxus, 
(EHLERTELLA, in others quite primitive and unchanged, e. g., 
Sonizocrania. These are all orbicular and highly chitinous 
shells; the line along which they have been derived departed 
at an early period from the radicle stock, apparently attained 
its diverse variations in Palaeozoic time, and has been continued 
to the present in the most highly modified representatives of the 
* This name was originally printed Darkautia, probably a typographical error in the spelling 
of the first syllables. 
166 
