74 ANNUAL REPORT OF 



the descriptions of the head plates of Semionotus ' given by- 

 European authors," but offered on his own part no new infor- 

 mation as regards the head structure of American forms. Only 

 within the past year has a really satisfactory figure of the head 

 of an American species been published, and this, which we owe 

 to Dr. G. F. Eaton, is unaccompanied by a textual description. 

 In the following paragraphs it is not intended to' present more 

 than a general sketch of the cranial structure so< far as it has 

 yet been deciphered. 



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 is frequently the case amongst the Semionotidse, these pairs 

 are not quite bilaterally symmetrical, but the sutures are more 

 regular than in some other genera. Skirting the lateral border 

 of the frontals, and extending also over the forward part of 

 the parietals, are deep mucous canals, which are developed on 

 the under side of the bones, and hence not commonly apparent 

 from the external aspect. In 5\ nilssoni (Fig. 10), the impres- 



Fig. io. 



Semionotus nilssoni Ag. Dorsal aspect of head, slightly modified, after 

 Agassi, X Vi- a, frontals ; b, parietals ; c, squamosal ; d, postorbital. 



sions of these canals are unusually broad and deep, and so diffi- 

 cult to distinguish from sutures that Agassiz was misled into- 

 confusing them with the latter. For the benefit of those who 

 may care to consult Agassiz's description of the latter head in 

 this form, and seek acquaintance with a single perfectly preserved 

 individual before attempting the decipherment of imperfect ones, 

 we quote from Agassiz's original description as follows : 



