Reports & Proceedings — Geological Society of London. 279 



common, while extreme forms were rare, the height being distributed, 

 iu fact, according to a typical ' variation curve '. If more than one 

 species were really present, it is in the highest degree improbable 

 that the various types should be distributed in the proportions 

 actually found, and this is taken as the most satisfactory proof possible 

 of the specific unity of the group. 



It is shown that the shells also vary extensively in respect of the 

 amount of carination, the degree of involution, the form of cross- 

 section of the whorls, the form of aperture, and the stage of 

 development at which various characters are acquired, the variation 

 in each character being, however, ' continuous.' The ontogeny of 

 the various characters is considered, and the species is shown to be 

 highly variable at an early stage, the mature characters being largely 

 independent of early variations. 



A. discussion is given of the bearing of the inquiry on the 

 stratigraphical application of palaeontological data. 



2. " The Structure and Relationships of the Carhonicolce." 

 By Miss M. CoUey March, M.Sc. (Communicated by Dr. G. 

 Sickling, F.G.S.) 



The evidence for the relationship of the Carbonicolce to the Unionidae, 

 based on shell-structure, muscle-scars, form, habitat, ligament, and 

 hinge-teeth appears insufficient. The first five of these characteristics 

 are shared with obviously unrelated groups. With regard to the 

 last — the teeth — as seen in developed specimens and reconstructions 

 from sections, they seem to be absent. The hinge-apparatus appears 

 to consist of a cardinal plateau, grooved for the reception of an 

 internal ligament. The hinge-plate and ligamental groove are absent 

 in Carhonicola antiqua, very poorly represented in C. turgida and 

 similar forms, and most highly developed in C. aquiUna. Another 

 fact which argues against the relationship of the Unionidae to the 

 CarlonicolcB is the absence of ornament in the latter group and its 

 presence in the former. This holds good whether the ornament is 

 considered to be due to the efi'ect of the glochidial hooks in the young 

 shells, or to be the remains of ornament ; because in the former case 

 it implies the absence of the glochidial stage, and in the latter it 

 implies descent from an unornamented ancestry. 



The position of the Carbonicolce appears to be unsettled, and to be 

 possibly quite different from that of any of the Pelecypoda yet 

 studied; because, according to Bernard's work, the cardinal plateau 

 is developed subsequently to the hinge-teeth, while in the CarlonicolcB 

 it is acquired independently of them, specialization taking place in 

 the ligament, for the reception of which the plateau is developed and 

 specialized. 



(ii) April 2S, 1913.— Dr. Aubrey Strahan, F.R.S., President, 

 in the Chair. 

 Professor E. Hull described the large chart of the North Atlantic 

 Ocean which was hung on the wall. Ttiis chart had been prepared 

 to illustrate the paper which he had read at the meeting of the 

 International Zoological Congress held at Monaco in March, to show 

 the mode of migration of animals by land-connexion between Europe 



