410 



THE GEOLOGIST, 



investigali)!?, much still remains to be discovered and satisfactorily 

 explained before many of the problems relating to the character and 

 history of the class can be considered as finally determined. 



All palaeontologists seem to agree that the Brachiopoda should be 

 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 un-artkulated, have their valves kept in place by means of 

 muscular and other contrivances. 



The articulated genera have been provisionally arranged into five 

 families — TerebratuUdce, Spiriferidce, Elajnchonellidce, SfropJiomenidw, 

 and ProcZwci^icZce— while the un-articulated comprise the Craniadcc, 

 Discinidce, and LinguUdce. But it is not my intention to enter 

 upon the important anatomical features which distinguish tbe 

 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 lamella3 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, 'Nov 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 which 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. 



!N"o 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 Rliynclionella 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 lamellae 

 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 Eoft parts of the oral arms. 



Dr. Gratiolct is of opinion that the median arm of Terebratula is in 



