410 
THE GEOLOGIST. 
invcstigali )r, much still remains to be discovered and satisfactorily 
explained before many of the problems relating to the character and 
history of the class can bo considered as finally determined. 
All palajontologists seem to agree that the Brachiopoda should bo 
divided into two principal groups. The first will include all those 
genera and species which have their valves articulated by means 
of teeth and sockets; the second will comprise those forms which, 
being tin-articulated, have their valves kept in place by means of 
muscular and other contrivances. 
The articulated genera have been provisionally arranged into five 
families — TerebratuUdce, Spirifcridce, Rhynchonellidct', Strophomenidcef 
and Productida; — while the un^articulatcd comprise the CraniadcE, 
Discinidce, and Lingulidce. But it is not my intention to enter 
upon the important anatomical features which distinguish the 
two groups, nor to discuss the subject of their general clas- 
sification ; the present communication being intended solely to 
convey some information, old and new, on those genera which were 
provided with spirally-coiled lamella) for the support of the oral arms. 
My classification (as well as all others hitherto introduced) is, to a 
greater or lesser extent, artificial and provisional. Nor is it surprising 
that, while feeling our way in the dawning light which has but 
recently begun to rise over the numerous efforts that have been made to 
extricate this class from the chaos in Avhich it was plunged for so long 
a period, that we should sometimes have stumbled, and have been obliged 
to retrace our steps, or even to abandon certain conclusions which may 
have been considered, for a time, as established upon a stable foundation. 
No recent Brachiopod hitherto discovered, has been found to possess 
calcareous spiral processes, so that we cannot determine by a direct 
anatomical examination, the exact relation of these calcified appendages 
to the soft parts of the oral arms ; but, as these are still existing 
among the articulated genera, in two species of Rhynclionella in which 
the fleshy arms, although free and unsupported, are spirally coiled 
and directed inwards towards the concavity of the smaller or dorsal 
valve, in a very similar manner to the broad spirally-coiled lamellaj 
in the extinct genus Atrypa, we may, to a certain extent, be allowed 
to conjecture upon the probable relations of the calcareous processes 
to the soft parts of the oral arms. 
Dr. Gratiolet is of opinion that the median arm of Terebratula is in 
