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- 
branc.hiate 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 haloliinnic 
forms. Spekia would in many ways appear to be very like 
a primitive Rissoa. 
Turning now to the last, and perhaps the most interest- 
