BRACHIOPODA. 133 



IC 



Neither can it be assigned to Okbiculoidea without opening that "enus to t 

 reception of heterogenous forms. For convenience, therefore, it is desirabk^ to 

 indicate the close but subordinate relations ol' this species* to Orbiculoidka, 

 by the use of the new term ^hlertella. 



The muscular impressions in Orbiculoidka are extremely faint and rarely 

 discernible at all. We have seen no specimens showing them as clearly as the 

 pedicle-valve of D. nitida, figured by Mr. Davidson and referred to above, where 

 they are resolvable into anterior and posterior adductors, situated close together 

 near the extremities of the pedicle-area, the posterior pair being at the distal 

 extremities of long progressive scars radiating from near the apex. This 

 arrangement of the scars indicates a general agreement with the muscular 

 anatomy of Discinisca lamellosa. 



Orbiculoidea Randalli, Hall, a remarkably large species, known only from a 

 single rather imperfect interior of the pedicle-valve, from the Hamilton group 

 of New York, shows a slight median septum passing from the apex forward, 

 comparable to that in Discinisca lamellosa; from this radiate a series of furrows, 

 probably of vascular nature, which lie only within the inner lamellae of the shell, 

 apparently not interrupting the external ornamentation. It will be interesting 

 to learn whether this shell is congeneric with Orbiculoidea Morrisi, Davidson. 



Occasionally, in the brachial valves of American species, are seen two 

 extremely faint ridges beginning near the apex and approaching each other at 

 their anterior termination, nearly meeting the inconspicuous median septum. 

 (See Plate IV f, fig. 22 ; also, Barrande's figure of Discina reversa, Keyserling, 

 Systeme Silurien, vol. v, pi. 95, fig I, 2 a.) In a large form from the Hamil- 

 ton group of New York, usually confounded with 0. grandis, Vanuxem, these 

 characters reach an extraordinary development aud become most prominent 

 features of the interior, beginning just in front of the apex, and, as they con- 



* The mode of preseivalion of this species freijuently leads to deception in regard to its pedicle-charac- 

 ters. When the two valves are preserved togetht^r, the breaking away of the upjiei' ]iorlion of the larger 

 valve often leaves its marginal portion surrounding the lower valve and closing the pedide-aperture. In 

 many such cases it is extremely clifficult to distinguish the parts belonging to the respective valves on ac- 

 count of the thinness of the shell, and we have taken pains to accumulate a very lai-ge representation of the 

 species in oi-der to fortify our conclusions. For much of the material at our disposal we are indebted to the 

 favor of Professor C. L. Herrick, of Cincinnati. 



