THE MOLLUSCS OF THE GREAT AFRICAN LAKES. 195 



Typhobias and the other members of the Halolimnic group 

 may really be. 



It will have been seen from the foregoing anatomical descrip- 

 tion that unless the family of the Melaniidse^ is to be considered 

 as an utterly heterogeneous group, the Typhobias are structurally 

 near, if they are not at, the opposite end of the whole Taeiiio- 

 glossate series. The Melanias, as they stand at present, are 

 certainly by no means homogeneous, and as Bouvier very 

 justly remarks, "La famille est une des plus mal etablies dans 

 tout le groupe des Prosobranches," but they do contain a sub- 

 stratum, possibly a majority of naturally associated forms, 

 and although it will be most important, when dealing with 

 other Halolimnic molluscs, to set limits to this group, the 

 question of its heterogeneity does not obtrude upon the present 

 discussion, the Typhobias being sufficiently distinct to be at 

 once dissociated from all those Melanian forms which have up 

 to the present time been anatomically examined. Whether 

 they may have relations among those numerous so-called 

 Melanias, the anatomy of which is utterly unknown, need not 

 be discussed. 



The unique and characteristic nervous system of Typhobia 

 at once dissociates this form from all the ordinary fresh-water 

 types. The great subintestinal ganglionic cord presents no 

 analogy even to the zygoneurous types of nervous system, such 

 as those of Potamides, Cerithidea obtusa, and Pyrgus 

 sulcatus, which have been rightly regarded by Bouvier and 

 others as representing the transitional links between the Palu- 

 dina, Bythinia (?), and true Melanian types of nervous system 

 on the one hand, and Haller's generally marine '* longicommis- 

 surate ^^ families on the other. In Typhobia Horei the 

 pleuro-subintestinal cord is in a most extraordinary condition, 

 at once primitive, specialised, and unique. It is specialised in 

 having lost the left pallial anastomosis, being thus neither 

 zygoneurous nor dyaloneurous on the left side, a condition of 

 things which finds its only parallel in the rather doubtful 



' See Bouvier's descriptiou of nerves of Melania, ' Ann. des Sci. Nat.,' 

 ser. 7, 1887, pp. 125—131. 



