134 COOKE: GEOGRAPHICAL DISTRIBUTION OF MOLLUSCA. 



The Comoro Islands. — This isolated group of islands, lying at 

 the extreme north of the Mozambique channel, and about midway 

 between Madagascar and the mainland, exhibit sufficient peculiarity 

 in their fauna to warrant their being considered apart from 

 Madagascar. About one hundred species are known, almost all of 

 which are peculiar. The principal feature is the rich development 

 of Ennea (30 species). On the whole the group shows rather more 

 relationship to Madagascar than to the mainland. Thus we have six 

 species of true Cyclostoi/ia, and only one Acliatiua, while among the 

 fresh-water genera is Sepfaria, which is characteristic of the whole 

 Malagasy sub-region, but is absent from the mainland. The 

 Helicidcc are all of insignificant size. It is interesting to note that 

 the Comoros furnish a good instance of the rule that operculate 

 genera are almost invariably widely distributed, while the genera of 

 Helix are not unfrequently very restricted ; thus the Madagascan 

 Cydostomata occur, not only on the Comoros, but on all the 

 adjacent groups and even on part of the mainland, while the Helices 

 are absolutely restricted to Madagascar itself. Peculiar to the group 

 is the remarkable genus Cyclosuri/s, an operculate shell of which 

 only the two first whorls are spirally coiled, while the rest are 

 produced in a long tube. Cyclotopsis and Geostilbia indicate affinity 

 with the Palrco-tropical region. 



MOLLUSCA OF THE COMOROS. 



Assiminca 3 



Melania 2 



Cyclophorus 4 



Cyclosurus 1 



Cyclostoina 6 



Otopoma 4 



Cyclotopsis 2 



Sepfaria 2 



(l>) The Mascarene Sub-region. — The Mollusca of the 

 Mascarene Islands (Mauritius, Bourbon, Rodriguez) and of the 

 Seychelles are thoroughly well known, and form an interesting group, 

 quite distinct from, though related to, those of Madagascar. There 

 are recorded from Mauritius 113 species, from Bourbon 45, from 

 Rodriguez 23, and from the Seychelles 34, a certain number of sub- 

 fossil species being included and " introduced " species excluded." 



* Helix aspersa and Achatina/ulica have been introduced into Mauritius and the Seychelles ; 

 H. similaris into Mauritius and Rodriguez, H . pulchclla and Ack. panthera into Mauritius 

 alone ; Ennea bicolor into Bourbon and the Seychelles. 



