360 GENERAL SUMMARY TO 



In the species above named the connecting lamella is proportionally larger and 

 somewhat reflected. 



bth Genus. — Glossothyris, Douville ; type T. nucleate/,, Schlotheim. 

 This so-termed genus seems to be based upon very deceptive exterior characters. 

 Its loop is short and similar to that of Terebratula. In external shape Terebratula 

 nucleata much resembles the recent Terebratula Wyvillei, Dav. External shape 

 cannot be taken into consideration, otherwise we would have to make as many genera 

 as there are species. Glossothyris is, according to my view, a synonym of Tere- 

 bratula. 



6th Genus. — Pygope, Link, 1830 ; type T triquetra, Parkinson (Anomia diphya, Colonna). 

 Douville commences by informing us that the loop is not completely known, but that 

 it is short, and that it does not differ materially from that of his genus Glossothyris. In 

 1869, in the ' Quarterly Journal of the Geological Society,' I described and figured the 

 loop of Terebratula diphya, and of this perhaps M. Douville was not aware at the time 

 when he wrote his memoir. 



1th Genus. — Dielasma, King ; type T elongata, Schlotheim. 



Douville admits that the loop is the same as in Terebratula, Klein. In 1850, King 

 erroneously placed T. elongata in Phillips's genus Epithyris, but removed it from that genus 

 in 1859, proposing for its reception a genus, Dielasma ('Dublin Natural History 

 Review, 5 vol. vi, p. 519, 1859), on account of the dorsal valve possessing dental 

 plates. 



Dr. Waagen, who warmly advocates the adoption of this genus, says that " the characters 

 of Dielasma are very easy to be recognised. They consist in the general terebratuloid 

 form of the shell ; a short loop like in Terebratula, fastened near its origin to the bottom 

 (hinge-plate) of the smaller or dorsal valve by two plates, which extend in a more or less 

 oblique direction from the sockets for the reception of the cardinal teeth towards the 

 middle line of the dorsal valve, forming sometimes a septum. The umbo of the smaller 

 valve bears a distinct cardinal process. In the larger or ventral valve the cardinal teeth 

 are supported by very strong dental plates. The existence of these dental plates, as 

 well as the septal ones, can easily be made out by scraping off a little of the shell on the 

 beak as well as in the umbonal region of the smaller valve ; the plates will then soon appear 

 as dark lines converging towards the termination of the beak as well as towards the apex 

 of the smaller valve." According to the same author, the genus seems to commence in 

 the Devonian period, and to continue through the Carboniferous and Permian formations; 

 it is rare in the Trias, and in the upper region of this system, the Kossen beds, it is 

 entirely replaced by Bhatina, Waagen, and Zugmeycria, Waagen ; so that in these beds no 

 traces of Dielasma are any longer found. Also in the Jurassic and Cretaceous times the 



