260 DUBLIN UNIVERSITY 



supported by plates attached to the dental protuberances, from which 

 they pass perpendicularly or obliquely : in the latter case they become 

 confluent, forming a sternum-like process, which is generally supported 

 by a perpendicular plate passing considerably forward along the medio- 

 longitudinal line of the valve. 



I include in this family the genera Waldheimia (the type), Terebra- 

 tella, Kingena, Ismenia, Meganteris, and some others, to which may be 

 added the new genera next to be described. Comparing the animal of 

 Terebratulina caput- serpentis with that of Waldheimia Australis, it is 

 difficult to conceive that there are not differences manifested, especially 

 in their respective labial appendages, of more than generic value : further, 

 the presence of pedicle muscle plates in the latter, and their absence in 

 the former, are also strongly in favour of this view. The confluent mus- 

 cular fulcral plates, in the small valve of most genera, have much resem- 

 blance to a bird's sternum, viewed on its inner surface. 



Genus. — Dielasma {King). 



Diagnosis. — Smooth, longitudinally oval, inequivalve, the forami- 

 nated or condyle valve being the largest : foramen complete : umbonal 

 cavity of the large valve furnished with muscular fulcral plates passing 

 perpendicularly from the dental protuberances to the surface of the valve : 

 umbonal cavity of the small valve furnished with muscular fulcral 

 plates, oblique, confluent, and forming a sternum-like process, supported 

 by a medio -longitudinal plate. Loop in type species short, slightly 

 recurved, and extending to about a third of the length of the valve : per- 

 forations in the shell tissue small and approximate. 



Type species. — Terebratulites elongatus (Schlotheim) I have long 



considered that the above species, which is one of the fossils charac- 

 teristic of the Permian system, ought to be separated from Terebratula, 

 the genus in which it is usually placed. In my " Monograph" I 

 made use of Professor Phillips's name "Epithyris" for it; but this has 

 been objected to, with some reason, by several parties. On similar 

 grounds, I object to the name Semiluna, which has been proposed for 

 the group by Professor M'Coy, considering that it was originally applied 

 to species which there is little doubt belong to the genus Ehynchonella. 

 I was therefore induced to apply to the above species the generic name 

 "Dielasma" in my "Historical Account of the Invertebrata belonging to 

 the Permian Rocks of the North of England," lately published. 



