ZOOLOGICAL AND BOTANICAL ASSOCIATION. 261 



Notwithstanding the small size of the loop in the present genus, as 

 compared with that characteristic of its congeners, I think the presence 

 of highly developed muscular fulcral plates warrants its removal, not 

 only from other short-looped groups generically, but its being consi- 

 dered a member of a distinct family. No known genus of Terebratu- 

 lidae, as restricted in the present paper, possesses any plates of the kind : 

 these structures are only to be seen coexisting with the long-loop in 

 genera belonging to the family Waldheimidee. The short-loop in Die- 

 lasma may therefore be considered as an exceptional character ; but there 

 is no ground for considering it as exceptional in anything else but in 

 length, inasmuch as it must be admitted that a short-loop may support 

 labial appendages throughout their entire extent as well as a long one. 



Dielasma is a protozoic form, occurring in Permian and Carboniferous 

 rocks. It appears to have lived also at a later period, as Terebratula 

 ovoides (Sow.) seems to be a Liassic representative of the genus. It is a 

 remarkable fact, that we are not yet acquainted with any other Ancy- 

 lobrachiate Palliobranch of the primary periods, except such as belong 

 to Waldheimidae, or are related to this family; for example, Meganteris 

 and Stringocephalus. 



Genus. — Macandeevia (King). 



Diagnosis. — Smooth, longitudinally oval, inequivalve, the condyle 

 valve being the largest : foramen emarginated by the deltidial fissure : 

 umbonal cavity of large valve furnished with two muscular fulcral 

 plates passing somewhat perpendicularly from the dental protuberances 

 to the surface of the valve : umbonal cavity of opposite valve also fur- 

 nished with similarly directed plates : cardinal muscular fulcrum exca- 

 vated in the substance of the hinge. Loop long, strongly recurved, and 

 extending in front of the centre of the valve. Perforations in shell tissue 

 distinct, and separated by interspaces equal in size to themselves. 



Type species. — Terebratula cranium. — Of late the shell which serves 

 as the type of the present genus, named in compliment to Mr. M 'An- 

 drew, has been considered a "Waldheimia, on account of its loop re- 

 sembling the type species of the genus just named; but some other 

 characters, apparently overlooked, necessitate, in my opinion, its removal. 

 In none of the figures published of Macandrevia cranium are the mus- 

 cular fulcral plates correctly represented : indeed, the fulcra of the large 

 valve appear to have been overlooked : those in the opposite valve are 



