58 CONNECTICUT GEOL. AND NAT. HIST. SURVEY. [Bull. 



with a narrow arched preoperculum. Ribs ossified. Fulcra un- 

 usually large. Paired fins small, dorsal fin large, arising at or 

 behind the middle of the back, and in part opposed to the rela- 

 tively small anal; caudal fin slightly forked. Scales smooth 

 or feebly ornamented, and the narrow overlapped margin pro- 

 duced at the angles and at the superior border. Flank-scales 

 not more than twice as deep as broad, the dorsal ridge-series 

 of acuminate scales forming a prominent crest. (Woodward.) 

 The cranial osteology of this genus is much more satisfactorily 

 known than that of Catopterus, although information is still 

 lacking in some particulars. The researches of Agassiz ac- 

 quainted us in a general way with the structure of the head 

 portion in S. nihsoni, from the Rhaetic of Sweden (see Plate 

 VI), and in recent years our knowledge has been increased by 

 the studies of E. Schellwien, Stromer, Tornquist, and other 

 foreign writers, and by the careful work" of Dr. George F. Eaton 

 of Yale in this country. Principal enlightenment has been gained 

 from investigation of S". nihsoni, bergeri, cap en sis and agassizii. 

 The more important cranial features may be briefly indicated as 



follows : 



The membrane bones of the cranial roof form a continuous 

 shield, extending from the snout nearly to the occipital border. 

 The two principal pairs of bones are the narrow and elongate 

 frontals, reaching from the premaxillaries to behind the orbits, 

 and the much shorter parietals in contact with them posteriorly. 

 As a rule these pairs are not quite bilaterally symmetrical, but 

 the sutures are more regular than in some other members of the 

 same family. Skirting the lateral borders of the frontals, and 

 extending also over the forward part of the parietals, are deep 

 mucous canals, which are developed on the cerebral side of the 

 bones, and hence not commonly visible in the outer aspect. Be- 

 hind the parietals occur a pair of wedge-shaped plates corres- 

 ponding to the supratemporals of Palaeoniscoids. These are 

 followed in turn by the squamose posttemporals, which in most 

 species resemble the like-named parts in primitive Chondrosteans. 



The squamosal is a plate of variable width and irregular shape 

 abutting against the parietals and frontals. It is terminated 

 anteriorly by a ring of circumorbitals, but its posterior limits 

 are apparently not the same for all species. The circumorbitals, 



