380 



Terebratulasina- 



-MOIJ.USCA.- 



-DlSCINID^. 



Tkiee I.— TEREBRATULANINA. 



The species belonging to this tribe liave the shelly 

 loop attached to the hinge plate, and is composed of 

 three genera, Terebratula, Terebratulina, and Wald- 

 heimia. 



Tribe II.— MAGASINA. 



In this tribe the species have tlie shelly loop at- 

 taclied to the septum, in the middle of the dorsal 

 valve, and is composed of six recent genera— Kre- 

 bratella, Magas, Bouchardia, Megerlia, Morrisia, and 

 Kraussia. 



Order II.— CRYPTOBKACHIA. 



Family— THECIDEID.S;. 



This is the only family of recent shells contained in 

 the order. The animals have the oral arms folded 

 upon themselves, united by membrane, fringed with 

 long cirrlii on their outer margins, and supported by a 

 shelly loop. The mantle extends to the margin of the 

 valves, and adheres closely. 



Ge.nus Argiope.— This genus has the loop folded 

 into two or four lobes, and adhei'ing to the prominent 

 septa with which the dorsal valve is furnished in- 

 teriorly. 



Genus Thecidea. — This genus has an unsymme- 

 trical loop divided into two or more lobes which are 

 united more or less intimately with grooves formed in 

 the disc for their reception. 



Sub-class II.— HELICTOPODA. 



This sub-class consists, like the preceding, of two 

 orders— the Sclerobrachia and the Sarcobrachia, 

 the first having the oral arms, suppoited by a shelly 



band arising from the hinder or cardinal edge of the 

 ventral valve ; the second, having the oral arms fleshy 

 to the base, and without any shelly support. 



Order I.— SCLEROBRACHIA. 



In this order there is only one family of recent shells, 

 and this consists of a single genus. 



Family- RUYNCHONELLIDiE. 



The Eh)Tichonellas have elongate, fleshy, spiral, oral 

 arms, supported at the base by two short, hard, diverg- 

 ing shell- lamiuffi arising from the hinge margin of the 

 ventral valve. 



Genus Rhynchonella. — This genus has a trigonal 

 shell, usually plaited, and acutely beaked. Externally it 

 is not punctate, and internally it has no testaceous skele- 

 ton or apophysary system. The dorsal valve is elevated 

 in front, and depressed at the sides, whilst the ventral 

 valve is hollowed along the centre. The foramen is 

 triangular and open, situate under the beak. Only 

 two recent species are known ; one from the northern 

 seas, and the other from New Zealand. 



Order II.— SARCOBRACHIA. 



This order is composed of three families of recent 

 shells, all distinguished by there being no processes on 

 the hinge-margin or disc in the lower valve, though in 

 some of them there is a slight medial longitudinal 

 elongation. 



Family— CRANIID^. 



The Cranias have the oral arms fixed to a process 

 of the ventral valve. The shell of Crania is orbicular, 

 calcareous ; tlie dorsal or upper valve of a conical 

 form, and the lower or ventral valve entire, flut, and 

 attached by its outer surface to marine bodies. Five 

 recent and twenty-eight fossil species are known. The 

 recent ones are natives of the north seas, the coasts of 



Great Britain, the 

 South Wales. 



Mediterranean, India, and New 



Family— DISCINID^. 



The Discinas, like the preceding family, have the 

 oral arms fixed to a process of the ventral valve ; but 

 the shell is horny, and when fresh and moist, is quite 

 flexible, and is attached to marine bodies by a short ten- 

 dinous peduncle, which passes through a small slit in 

 the disc of the lower valve. The upper valve of Dis- 

 cina is conical, and resembles the shell of a limpet or 

 patella, the lower is orbicular and flat ; they are not 

 united together by a hinge, and the external surface is 

 minutely punctate. Seven recent species are known, 

 chiefly from South America. 



