6y6 



MOLLUSCOIDEA. 



valve is provided with one or more sub-marginal septa, with which 

 the brachial loop becomes more or less extensively fused. The 

 shell-structure is punctate. The range of the family is from the 

 Jurassic period to the present day. 



The type-genus of this family is Argiope itself, which ranges from 



the Upper Jurassic to the present 

 day. In this genus the shell (fig. 

 546) is transversely oval, with a 

 straight hinge-line and an area to 

 each valve, and having a large fora- 

 men, with a rudimentary deltidium. 

 The interior of the dorsal valve is 

 furnished with from three to five 

 sub-marginal septa (s) ; while the 

 brachial loop follows the margin of 

 the shell, and becomes attached to 

 the septa, thus becoming four-lobed, being at the same time partially 

 confluent with the shell. Cistella (Lias to Recent) closely resembles 

 Argiope in general characters, but there is only a single septum in 

 the dorsal valve, and the brachial loop is two-lobed. 



Family 9. Stringocephalid^e. — In this family the shell is 

 rounded ; the beak of the ventral valve possesses a foramen and 



Fig. 546. — Interior of the dorsal valve 

 of Argiope decollata (Recent), enlarged, 

 showing the sub-marginal septa (s) and 

 the brachial loop (/). (After Davidson.) 



Fig- 547- — A > Interior of the dorsal valve of Stringocephalus Burtini, Middle Devonian ; b, 

 Shell of the same viewed in profile and showing the interior : a, Adductor impression ; p, Cardinal 

 process ; h, Hinge-plate ; c, Crura of loop (/) ; d, Dorsal septum ; z>, Ventral septum ; t, Dental 

 sockets. The radial processes of the loop are shown in a by dotted lines. (After Davidson.) 



deltidium ; the cardinal process of the dorsal valve is greatly de- 

 veloped ; and the brachial loop is marginal, and carries internally- 

 directed radial processes. The only genus comprised in this family 

 is Stringocephalus itself, which is confined to the Silurian and De- 

 vonian rocks. 



