of some species of Cyclostomacea. 209 



valuable light on the geographical {listributiou of genera. The 

 tropical island of Borneo, as yet scarcely explored beyond its 

 shores, appears to be a centre of production of the Oriental 

 Cyclostomacea. In it we find representatives of Cyclophorus, 

 Pterocyclos, Opisthoporus, Cyclotus, Leptopoma, Diplommatina, 

 Alycteus, Rhaphaulus, Megalomastoma, Hydrocena, and Ompha- 

 lotropis. To the north-cast it sends out an offset to the Philip- 

 pines, where Leptopoma and Cyclophorus abound, and Megalo- 

 mastoma, Cyclotus, Pupina and its allies appear more sparingly. 

 To the south-east, Cyclophorus, Diplommatina, Hydrocena, and 

 Pupina proceed along lines extending to the Louisiade Archi- 

 pelago, New Holland, Lord Howe's Island, and New Zealand. 

 Towards the north-west, with which we are more immediately 

 concerned, the chief branch runs up the Malay Peninsula, first 

 appearing at its extremity, in Singapore, in Cyclophorus, Hydro- 

 cena, and Pupina?; at Malacca, in Cyclophorus; and at Pulo 

 Pinang and its vicinity, in the same genus, Leptojjoma, and 

 Rhaphaulus. In the Tenasserim Province the family assumes a 

 great development in two or three species of Cyclophorus, Lep- 

 topoma, Pterocyclos, Otopoma^, Pupina (2 sp.), Megalomastoma 

 (2 sp.), Rhaphaulus, Alycceus (2 sp.), and Hydrocena. In Pegu 

 and Ava we find eight species of Cyclophorus, besides Lejtto- 

 poma, Pterocyclos, Alycceus (3 sp.), and Hydrocena (2 sp.). 



The next point explored to the northward is the Khasia range, 

 where we have seven Cyclophori, after deducting a variety and a 

 species which has been erroneously ascribed to the tract ; one 

 Leptopoma, three Pterocycli, two Diplommatin(2, two Alyaei, 

 one Pupina, and two Hydrocence. Following the mountain- 

 chains, round the head of the Assam Valley, to the Himalayan 

 ridge, we come upon Megalomastoma in Bhotan; and in Sikkim 

 the family puts forth a great effort in the production of four 

 species of Cyclophorus, one Diplommatina, three Alyccei, one 

 Megalomastoma, and one Streptaulus. Passing the (concholo- 

 gically) unexplored region of Nipal, the Western Himalaya 

 makes an expiring sign in a single Cyclophorus, three Diplom- 

 matina, and an Alycceus. 



On reviewing the above enumeration of genera and districts, 

 I do not find a single species reproduced in a second tract ; 

 each has its own peculiar representatives, — a rule which does 

 not hold good with regard to the Helicidae, inasmuch as I find 



* Otopoma Blennus (Annals, vol. xvii. p. 231) has not the solid struc- 

 ture of O. clathratulum, nor the descending last whorl ; its aperture is less 

 oblique and larger, and the umbilicus is narrower ; the operculum, lately 

 forwarded, proves to be calcareous, somewhat concave, and with five gra- 

 dually increasing whorls on the exterior side, while the inner surface is 

 rather convex, with a central umbo, and only U rapidh increasing turns. 

 Ann.S^' Mag.N.Hist. Sev.2. Vol.'xix. U 



