On the Structure q/* Magelona. 147 



By the addition of the valuable materials obtained by one of the 

 naturalists of the " Transit-of-A"euus" Expedition to Eodriguez, 

 and by the Hon. Edward Newton in Mauritius, as well as by the 

 aid of supplementary information received from other sources, the 

 author has been enabled to show in the present parts of his paper 

 that the round-headed division of Tortoises is confined to Aldabra 

 and never extended to the Mascarenes proper, and that the Tor- 

 toises from the latter islands can be externally, though not osteo- 

 lojgically, distinguished as a ivhoh from the Galapagos Tortoises, as 

 will be seen from the following synopsis : — 



I. Nuchal plate absent. ' Erontal portion of the skull flat. Fourth 



cervical vertebra biconvex. Pelvis \vith broad symphysial 

 bridge. 



A. Gular plate double ; sternum of 



moderate extent Galapagos Toetoises. 



B. Gular plate single ; sternum short . . Mascaee^te Toetoises. 



a. Carapace thin, thickened towards the margins ; 

 centre of the last vertebral plate raised into a 

 hump, which is separated from the penultimate 

 vertebral by a transverse depression : Tortoises 

 of Maurititis {T. triserrata, T. inepta, T. indica, 

 T. leptocnemis). 



h. The entire carapace extremely thin and fragile, all 

 the bones very slender: Tortoise of Rodricjue.z (T. 

 Vosmcfri^. 



II. Nuchal plate present. Erontal portion of the skull convex. 



Third cervical vertebra bicon-^'ex. Pelvis with narrow sym- 

 physial bridge. Gular plate double. Carajiace thick. Aldabra 

 Toetoises {T. elephuntina, T. Daudinii, T. ponderosa, T. holo- 

 lissa). 



Feb. 22, 1877.— Dr. J. Dalton Hooker, C.B., President, in the 



Chair. 



On the Structure of JSlarjehna. Bv AV, C. M'Intosh, M.D., 

 F.K.S.E., F.L.S. 



This annelid was first discriminated * by Dr. George Johnston, 

 of Berwick ; but as his description (under the name il/^a mirahilis) 

 \\as not published till 18G5, the above-mentioned title, given to the 

 same type, from the Island of St. Catherine, off the coast of 

 Brazil, by Dr. J^'ritz Miiller, has the priority. It is a compara- 

 tively small form, its' slender body being divided into two well- 

 marked regioiLS ; w hile anteriorly two long papillose tentacles are 

 attached to the base cf a remarkable spathulate, eyeless snout, 

 which it dextrously uses to perforate sand near low-water mark 

 at 8t. Andrews and other sandy shores on both east and west 

 coasts. 



* From specimens procured by tlie distinguished botanist, Dr. Greville. 



10* 



