142 PERMIAN FOSSILS, 



Genus Ismenia^ King. 



Diagnosis. — Uni-areagerous : wider than long : plicated : semicircular in front, 

 and angulated behind. Hinge line slightly inclined on each side of its centre. Area, 

 both halves oblique to the hinge-margin, and to each other. TJmbone projecting behind 

 the hinge-line, and truncated with Xhe, foramen, which is large, and closed inferiorly 

 by the deltidium. Punctures moderate in size. 



Type Terebratulites pectunadus, Schlotheim. 



This genus has much the appearance of Belthyridcea ; but it differs principally in 

 having the area and the hinge-line not straight, and the foramen not below the point 

 of the umbone. Ismenia posteriorly has a decided approach to certain Terehratulidce ; 

 whereas BeltJiyridcBa strongly simulates Spiriferidce. It appears to be a connecting 

 link between Bhynchorida and Terehratulidce. 



There appear to be other genera of the present family belonging to the secondary 

 rocks. 



Family Magasid^ {partini), A. d'Orbigny. 



The type of this group is the genus Mayas, J. Sowerby, which with Bouchardia,'^ 

 Davidson, are the only genera it appears to contain. I have some doubt as to either 

 of them possessing characters sufficiently distinctive to warrant their removal from 

 Terehratulida. 



Family ARGiopiDiE, King. 



This group, or rather its type Argiope, Deslongchamps i^Megathyris, A. d'Orbigny), 

 is included by Mr. Gray in the next family ; but both its external and internal cha- 

 racters are obviously opposed to the association. 



Family Thecid^id^e {partim), Gray. 



The completely distinctive character of this group is too generally admitted to 

 require any other than a passing allusion to it. Probably some modifications in 

 Thecidceidoe and Argiopidce, as far as they bear on their associate groups, will be 

 required when the animal belonging to their respective types is made known. 



^ Named after Ismene, a daughter of (Edipus and Jocasta. (Vide Lempriere's Bibliotheca Classica.) 

 * Vide Bull. delaSoc. G60I. de France, Dec. 1849. Certain characters of the type oi Bouchardia (viz., 



Terebratula rosea. Humph.) were briefly noticed by myself in the Annals and Magazine of Natural 



History, vol. xviii, pp. 34, 38, 1846. 



