274 THE TANGANYIKA PROBLEM. 



the Cerithoid nervous system and a feebly developed 

 radula, the teeth of which might with equal propriety 

 be regarded as approaching several diverse forms. In 

 this group the most remarkable feature which is exhibited 

 consists in the genital groove, which here leads into a 

 brood pouch in the foot, on the left side of the head, 

 a condition of things which may be simulated but is 

 not repeated in any other form hitherto examined. If, 

 however, the comparison which I have already drawn 

 between the groove and the brood pouch in Tanganyicia ; 

 the somewhat different but analogous arrangement in M. 

 episcopalis ; the fragmentary appearance of parts of this 

 apparatus in such forms as Strombus and Littorina ; and 

 the similar arrangement of grooves and introvertible penes 

 in Opisthobranchs, be correct, we must conclude that Tan- 

 ganyicia possesses an archaic character which may be 

 encountered in part, or in entirety, in widely diverse 

 molluscan types. See fig. on p. 273. Tanganyicia may thus 

 be said to exhibit the morphological characteristics which 

 we should attribute to those earlier members of the Proso- 

 branchiate group which anteceded the modern forms typified 

 by such genera as the Strombus and Cerithium of to-day. 



In Spekia we have a form which, in its nerves, is 

 unquestionably a naticoid, but which, in the absence of a 

 number of the more specialised naticoid structures, such 

 as the glandular masses related to the oesophagus, and so 

 on, is unquestionably simple ; and the same conclusion is 

 forced upon us, both by the early Cerithoid character of its 

 radula and possession of the archaic gastric apparatus 

 and crystalline style, characteristic of all the halolimnic 

 forms. Spekia would in many ways appear to be very like 

 a primitive Rissoa. 



Turning now to the last, and perhaps the most interest- 



