Notices of Memoirs — The British Association. 559 



2. — On the Lamellibranchtate Fauna of the Upper Helder- 

 BERG, Hamilton, Portage, Chemung and Catskill Groups 

 (equivalent to the Lower, Middle and Upper Devonian 

 OF Europe) ; with Especial Reference to the Arrange- 

 ment OF the Monomyaria and the Development and Dis- 

 tribution of the Species of the Genus Leptodesma. 



By Professor James Hall, LL.D. 



THE investigations of the fossil Laraellibranchiate shells has been 

 carried on as a part of the work of the palteontology of the 

 State of New York. Already ninety plates have been lithographed, 

 and these with their explanations giving the names of the fossils have 

 been distributed to the principal scientific societies of Europe and 

 America, The full text of the descriptions of the species of the 

 Monomyaria, 268 pages and plates i.-xxxiii. and Ixxxi.-xcii., have 

 been published complete. The remaining portions of the work were 

 well advanced. 



The Monomyaria are described under twenty-one genera and 284 

 species. The remaining portion of the work contains illustrations 

 of about 215 species under thirty-three genera. 



The author has found it necessary to make subdivisions among the 

 forms usually referred to Avicidopecten, and it has seemed equally 

 important to propose other generic names for forms which have here- 

 tofore been indiscriminately referred to AvicuJa, Pterinea, Pteronites, 

 etc. While the essential internal ciiaracters have been regarded as 

 of primary importance, such an arrangement has been made of the 

 species that the student may determine their generic relations from 

 the general form and exterior markings alone. Since, in all forms 

 of the fossil Lamellibranchiata the interior surface usually remains 

 attached to the matrix, a reliable means of identifying the genera by 

 external chai-acters becomes a consideration of primary importance. 



Among the new genera proposed, Leptodesma presents some 

 features in its development and distribution which may be of more 

 than ordinary interest. The upper part of the Chemung group 

 exhibits such physical features as might be expected from a gradually 

 shallowing sea and the approach of estuarine conditions. Numerous 

 circumscribed areas appear to have existed, and these, while often 

 characterized by an abundant fauna, contain few species, and these 

 forms are extremely limited in their geographical range. The species 

 of the genus Leptodesma are often abundant and very characteristic of 

 certain horizons within limited areas, but rarely have a general dis- 

 tribution through the strata, as some species of the Brachiopoda. 

 They seem to have been developed in shallow lagoons, and the 

 characteristic species of one of these areas rarely appear in another. 

 At the same time the physical condition or other causes have operated 

 to develope a remarkable variety in form, and as it does not seem 

 possible to separate these forms generically, it becomes necessary to 

 arrange them in distinct groups or sections. 



These sections have been made with reference to the most pro- 

 minent characteristic of the forms. Of those already known and 



