142 PERMIAN FOSSILS. 
Genus Jsmenia,' 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. Umbone projecting behind 
the hinge-line, and truncated with the foramen, which is large, and closed inferiorly 
by the deltidium. Pwnxctures moderate in size. 
Type Zerebratulites pectunculus, Schlotheim. 
This genus has much the appearance of Delthyridea; but it differs principally in 
having the area and the hinge-line not straight, and the foramen not below the point 
of the umbone. /smenia posteriorly has a decided approach to certain Terebratulide ; 
whereas Delthyridea strongly simulates Spiriferide. It appears to be a connecting 
link between Rhynchoride and Terebratulide. | 
There appear to be other genera of the present family belonging to the secondary 
rocks. 
Family MAGASID& (partim), A. dOrbigny. 
The type of this group is the genus d/agas, J. Sowerby, which with Bouchardia,2 
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 
Terebratulide. 
Family ARGIOPIDE, King. 
This group, or rather its type 47giope, Deslongchamps (= 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 THECIDEIDA (partum), 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 
Thecideide and Argiopide, as far as they bear on their associate groups, will be 
required when the animal belonging to their respective types is made known. 
1 Named after Isméne, a daughter of Cidipus and Jocasta. (Vide Lempriere’s Bibliotheca Classica.) 
* Vide Bull. de la Soc. Géol. de France, Dec. 1849. Certain characters of the type of Bouchardia (viz., 
Terebratula rosea, Humph.) were briefly noticed by myself in the Annals and Magazine of Natural 
History, vol. xvii, pp. 34, 38, 1846. 
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