1 76 HOLOCEPHALI 



and the pelvic girdle and fin more normal. Unfortunately, palaeon- 

 tology throws but little light on the ancestry of Chimaeroids ; for 

 the genera from Devonian rocks, which are only very doubtfully 

 referred to this Order (Ptyctodontidae), are much too imperfectly 

 known to afford any trustworthy evidence on the subject. 



GROUP A. 



Where known, the body is found to be covered with denticles, 

 the rostrum long and depressed, the tenaculum straight and pointed, 

 the dental plates thin and without well-defined grinding areas. 



Family SQUALORAJIDAE. Large denticles are scattered over the elon- 

 gated and depreased body. The head appears to have been flattened and 

 expanded with a long depressed rostrum. Above the latter was a long 

 movable tenaculum, or frontal clasper, armed with denticles (Fig. 141, B). 

 No dorsal fin spine is known. The tooth-plates, four above and two 

 below, are thin and without well-differentiated tritoral areas (Fig. 141, D). 



Squaloraja, Riley ; Lower Lias, England. 



Family MYRIACANTHIDAE. The head bears several tuberculated 

 paired dermal plates, of vasodentine, which may have projected from 

 the sides of the head and lower jaw (Fig. 142). There is a large com- 

 pressed rostrum with bent tip as in Callorhynckus. Long calcified rods 

 are generally interpreted as labial cartilages. The teeth, consisting of 

 paired palatine, vomerine, and prevomerine plates above, and two man- 

 dibular and a presymphisial tooth below, are thin and have ill-defined 

 tritoral areas (Fig. 141, A). A tuberculated dorsal-fin spine is present. 



Myriacanthus, Ag. ; Lower Lias, England. Chimaeropsis, Zittel ; 

 Lower Kimmeridgian, Bavaria. 



GROUP B. 



With a quite or almost scaleless body, and a short tenaculum 

 bearing denticles on its swollen extremity. The teeth become 

 thicker, and usually have distinct grinding patches (Fig. 141). 

 The dorsal fin spine is smooth. 



Family CALLORHYNCHIDAE. The rostrum has an expanded end. The 

 large teeth have well-marked tritoral areas. No calcareous rings are 

 present in the notochordal sheath. The cartilaginous support of the 

 mixipterygium is comparatively simple. The lateral -line canal is a 

 closed tube opening by pores in the adult. 



Callorhynchus, Gronow ; Pacific, and Cretaceous, New Zealand (Fig. 

 143). 



Family CHIMAERIDAE. The rostrum is quite short. Anterior tritors 

 are present on the teeth. The notochordal sheath has small calcified 

 fibro-cartilage rings ; and the clasper divides into three branches, each 

 with a cartilage axis (Fig. 139). The lateral-line organs are in an open 

 groove [281]. 



Chimaera, L. ; widely distributed iu the deep seas ; Pliocene, Italy 



