XXXII. CHIROSTOMA 69 



Chirostoma or its synonym, Atiierinichthys, proves on more critical 

 examination to belong to some other genus of the family. 



Even as thus restricted, Chirostoma comprises a diverse, and at 

 first consideration apparently unnatural, assemblage of species. We 

 regard them all as congeneric only because of the existence of several 

 species intermediate between each of the contrasting groups of species 

 which has been distinguished. A graded series of species may be 

 arranged in reference to each of the widely varying characters. The 

 size varies from small to large ; the vertebrae from 37 to 45 in number ; 

 the scales from 36 to 75 transverse series ; the scale borders from strictly 

 smooth to sharply crenate ; the origin of the first dorsal from a position 

 over that of the anal to a point as near the insertion of the ventral fin ; 

 the proportionate size of head, snout, and eye, from small to large ; 

 the elevation of the mandibular rami from slight to great; the projection 

 of the mandible beyond the tip of the premaxillaries from little to much; 

 the teeth from small to canine-like in size, and from a biserial to a band 

 arrangement. The sequence of species in these several series, however, 

 is not the same. Having determined these facts by an examination 

 of all the known species of the genus, we are unable to make a satis- 

 factory division of the group, diverse as it is. 



Chirostoma is very closely related to Menidia, from which it 

 dififers constantly in the projection of the lower jaw at its symphysis 

 beyond the upper,-* and the extension of the air bladder into the tail 

 over about one-third to two-thirds of the anal base. Other distinctive 

 features of Chirostoma, as compared with Menidia, are usually but 

 not always evident. These include the larger size; the development of 

 vomerine teeth ; the longer, more pointed pectoral fin ; the small size of 

 scales, their crenate margins, and the lack of circuli on their exposed 

 fields. From Thyrina, equally near, Chirostoma differs constantly in 

 the more anterior position of the first dorsal fin, and the projection of 

 the lower jaw; usually in the smaller size of the scales, etc. Some 

 species of Austromenidia resemble the finer-scaled species of Chi- 

 rostoma closely, even having the lower jaw as an individual variation 

 slightly produced beyond the upper. This condition in Austromenidia, 

 as Abbott pointed out, is due rather to the weakness of the upper jaw, 

 than to the increased strength of the lower jaw characteristic of Chi- 

 rostoma. The resemblance seems not due to close relationship. 



We distinguish 18 species of Chirostoma, two being named now 

 for the first time. We find no evidence that the species are not well 

 fixed, as intimated by Dr. Meek, and find but one instance of apparent 



2* Dr. Meek's figures of his species of Chirostoma show the lower jaw decidedly 

 too short, a fact which perhaps led Eigenrtiann to refer some of the species to 

 Menidia, and to doubt the generic distinctiveness of the two groups. 



