the Cyclostomacea of Eastern Asia. 451 



other instance of the affinity of the hill-fanna of Southern India 

 to Malayan types. 



9. Spiraculum, Pearson ; and 10. Opisthoporus, Bens. 



The genus Spiraculum, distinguished by the possession of a 

 retroverted sutural tube open at both ends, and by a modification 

 of the form of the mantle corresponding to the same, has, I 

 think, better claims to generic distinction from Pterocyclos than 

 even Rhiostoma. Its claim to separation from Pterocyclos I 

 believe equal to that of Opisthoporus from Cyclotus ; and I should 

 be inclined to class Opisthoporus as a subgenus of Spiraculum, as 

 I have little doubt that the animal of the former genus, when 

 examined, will be found to be similar in structure to that of the 

 latter. 



Mr. Benson has lately described, under the name of Opistho- 

 porus Gordoni, a species from Molmain, in Burma, which may, 

 I think, very possibly prove to be a Spiraculum, when the oper- 

 culum is obtained, as the known geographical range of forms 

 belonging to that genus approaches far nearer to Molmain than 

 that of Opisthoporus. To whatever type it may belong, it serves 

 admirably to illustrate the extremely close affinity between them, 

 since there is no character possessed by it which may not be 

 found in one or the other form of each genus. S. Avanum, W. 

 Blanf., has a duplicate lip and a smooth surface ; while an un- 

 described species occurring in Assam has the last whorl free for 

 a longer distance than in any known species of Opisthoporus ; 

 and it has also a more prominent projection from the upper 

 portion of the peristome. The sole difference between the two 

 genera is in the structure of the operculum. 



II. Rhiostoma, Bens. 



The projection from the upper portion of the peristome in this 

 species differs from that in Spiraculum and Opisthoporus in being 

 accompanied by a deep incision in the inner peristome, clearly 

 showing that it is the homologue of both wing and incision in 

 Pterocyclos, and of both projection and sutural tube in the first- 

 mentioned genera. In R. Housei, Pfr., the projection indeed 

 forms a perfect tube. Rhiostoma is doubtless a good type, 

 although species may very probably be met with connecting it 

 with Pterocyclos more closely than is now the case. A single 

 species {R. Haughtoni, Bens.) is found at Molmain ; all others 

 are Malayan. 



12. AuLOPOMA, Troschel. 



I have no especial remark to offer upon this genus, which has 

 not hitherto been met with out of Ceylon. 



