726 itEroRT— 1881. 



In 1883 the same author (' Zeitschrift der Deutschen Geologischen Gesellschaft,' 

 Bd. XXXV., p. 704) has discussed the relations of Tetragonis Eifelensis, with 

 Dictyophyton, describing and illustrating Dictyophyton (rerolsteineyisis. 



This group presents a great variety of form in the mode of growth. The 

 structure of the frond which characterises every member of this family, may he 

 described as a reticulation of tubular spicules forming rectangular meshes. In the 

 simpler forms these meshes alternate in size and strength, owing to the regular 

 alternation in the size of the hundle3 of spicules which determine the meshes. In 

 the prismatic and nodose forms, certain bundles of spicules become very much 

 developed and produce the characteristic form and ornamentation of the cup. The 

 middle layer is uniformly reticulate ; while the inner and superficial layer show au 

 oblique and sometimes a radiate arrangement of spicules. In the highly-ornamented 

 species, the outside layer of spicules is often produced into tufts, spines, and 

 intersecting fimbria or lamina? of greater or less prominence. 



In the foreign literature accessible to the writer, there are six species of this 

 family described. In preparing a memoir on the subject he has been able to 

 recognise, from personal examination, thirty-seven American forms, the oldest 

 geologically being from the Utica State, and the latest form from the Keokuk 

 limestone of the Carboniferous system. These thirty-seven species have been 

 described under the following genera, viz.: — Cyathophvcus, AValcott ; Dicty- 

 ophyton, Ectenodictya, Lyrodictya, and Physospongia, Hall ; and Uphantsenia, 

 Vanuxem. 



2. On the LamellibrancMata Fauna of the Upper Helderberg, Hamilton, 

 Portage, Chemung and Catshill 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 Distribution of the 

 Species of the Genus Leptodesma. By Professor James Hall, LL.D. 



The investigation of the fossil Lamellibranchiate sbells has been carried on as a 

 part of the work of the palaeontology 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 LXXXI.-XCTL, have been published complete. The re- 

 maining portions of the work are 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 Aviculopecten, and it has seemed equally important to propose other 

 generic names for forrns which have heretofore been indiscriminately referred to 

 Avicula, Pterinea, Pteronites, &c. While the essential internal characters have 

 heen 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 iden- 

 tifying the genera by external characters 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 he ex- 

 pected from a gradually shallowing sea and the approach of escuarine conditions. 

 Numerous circumscribed areas appear to have existed, and these, while often charac- 

 terised 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 distribution throughithe strata, as some species of the Brachiopoda. 



