NATURAL SCIENCES OP PHILADELPHIA. 79 



April 23d. 



Vice-President, Bridges, in the Chair. 



Thirty-one members present. 

 A paper was presented for publication, entitled 

 " Notes on the habits of Aphredoderus Sayanus, by Charles C. Ab- 

 bott," and was referred to a Committee. 



April oQtli. 



Mr. Lea, President, in the Chair. 



Twenty-three members present. 



On report of the respective Committees, the following papers were 

 ordered to be published : 



Revision of the Genera of North American SCI2EN1N2E. 



BY THEODORE GILL. 



The present memoir has resulted from our studies of the characters of the 

 Liostomi and other American Scirenoids, and was intended only to embrace the di- 

 agnoses of our genera, but in order to appreciate more fully the relations of those 

 forms, we were induced to study the foreign ones, and have believed that the 

 results are of sufficient interest and importance to submit to ichthyologists. 

 There is a number of other genera confounded under those of Cuvier, but as 

 a gentleman of the Museum of Comparative Anatomy of Cambridge is under- 

 stood to be engaged in the profound study of the whole family, we refrain from 

 naming and characterizing them. The diagnoses of the exotic genera here de- 

 scribed will be sufficient to enable the reader to appreciate the distinctions 

 which exist between our own species and those of the genera with which most 

 of them have been hitherto considered congeneric. 



Subfamily Scl.enin,e Gill. 



The body varies in shape, ranging from an oblong rhomboideo-ovate form to 

 an elongated fusiform one. When the inferior outline of the head ascends to 

 the snout, it is with a very gradual and slight curve. Both jaws are formed 

 with teeth, which are of a more or less acutely conical form. 



The dorsal fins are united by a slightly elevated membrane ; the first is of 

 moderate height, being longer than high ; the second is oblong or elongated. 

 The anal fin is of a trapezoidal form, and as high or higher than long. The 

 ventral fins are generally inserted under or behind the bases of the pectorals , 

 rarely a very short distance in front. 



The scales are ctenoid and generally arranged in very oblique rows. 



The inferior pharyngeal bones when in place form a triangular U or V-shaped 

 body with a broad triangular excavation, whose sides are slightly emarginated, 

 and whose anterior apex is suddenly continued to an oblong triangular fissure 

 between the opposite bones. Each bone is itself semi-claviform and more or 

 less curved upwards behind ; its external vertical margin is straight or nearly 

 so ; its internal margin, for the first half of its length, is also straight, but 

 thence, with a slight sigmoid curve, converges to the end of the posterior pro- 

 longation of the bone. Beneath the bone, there is an external ridge which is 

 marginal before but slightly recedes behind. 



The upper pharyngeal bones are six, or three on each side ; the median is 

 subtriangular or triangularly-ovate ; the external, or anterior and posterior 

 elongated in the direction of the sides cf the median ; the posterior is broadest, 

 and has a more or less subovate form. 



1861.] 



