5 1 6 Memoir Sears Foundation for Marine Research 



plates in upper jaw and by one pair in lower jaw; no cloaca; urinary papilla and genital 

 apertures posterior to anus;^ a pair of abdominal pores near anus, putting peritoneal 

 cavity into communication with exterior (as in sharks), but these are more or less ob- 

 literated during growth; males with an intromittent organ (clasper), simple or divided, 

 developed from base of each pelvic fin on inner side; a supplemental clasping organ or 

 prepelvic tenaculum close in front of each pelvic fin ; also a frontal clasper on forehead, 

 a structure unique among fishes; scales, when present, essentially similar to those of 

 elasmobranchs. 



Orders. The few modern chimaeroids resemble one another so closely in their 

 bodily make-up that they are grouped here in a single order, Chimaerae.* 



Relationship to Other Vertebrates. "The living chimaeroids are a divergent and modi- 

 fied branch of some primitive shark-like type. Beside certain characters of the bony 

 fishes, they have acquired others distinctively their own. Their relationship to the elas- 

 mobranchs is seen in their cartilaginous skeletons, dermal denticles, the brain structure, 

 and especially the reproductive organs. The large eggs and their enclosure in horny 

 coverings is another interesting feature in common. The single gill opening is modified 

 toward the bony fish type,"^ as is the presence of a dermal opercle covering the gill 

 opening as well as the fact that the anus opens externally in front of the urinogenital 

 apertures rather than into a cloaca. 



Order CHIMAERAE^ 



External Characters. Trunk, in all modern chimaeroids, more or less compressed 

 laterally, tapering rearward to slender tail. Snout either rounded-conical, extended as 

 a long pointed beak, or bearing a curious hoe-shaped proboscis. Two dorsal fins; first 

 dorsal triangular, usually higher than second and perhaps incompletely separated from 

 latter in some cases, edged anteriorly by a strong sharp-pointed bony spine' reaching 

 nearly to apex of fin or a little beyond; both spine and first dorsal capable of erection 

 and of depression into a groove in midline of back; second dorsal extending back nearly 

 to origin of caudal fin, either of uniform height throughout its length or partially or 

 wholly subdivided, not erectile or depressible. Caudal fin narrow, tapering, prolonged 

 in some as a long filament, either with a more or less definite lower anterior lobe and 

 with its axis somewhat raised (i. e., shark-like or heterocercal) or without lower anterior 



3. See Dean (Chimaeroid Fishes, Publ. Carneg. Instn., 32, 1906: pi. i, fig. 2) for illustration showing relative positions 

 of anus and urinary papilla in Hydrolagus colliei. 



4. In this we follow Jordan (Class. Fish., 1923: 105, Chimaeroidei), Goodrich (in Lankester, Treat. Zool., 9, 1909: 

 168, Holocephali), Romer (Vert. Paleont., 1945: 578, Chimaerae), Berg (Class. Fish., Engl. Trans., 1947: 383, 

 Chimaeriformes), and various earlier authors. The chimaeroids have recently been distributed by Fowler (Bull. 

 U. S. nat. Mus., 100 [73], 1941 : 487) between two orders, Chimaeroidei without proboscis, the males with bifid or 

 trifid claspers, and Callorhinchoidei with proboscis, the males with simple rod-like claspers. But this division takes 

 no account of the nature of the caudal fin, i.e., whether heterocercal or diphycercal, which seems to us more significant 

 from the evolutionary standpoint than either the nature of the snout or the secondary sexual characters. 



5. Fowler, Bull. U.S. nat. Mus., 100 (jj), 1941: 486. 



6. This name seems to have been employed first in an inclusive sense by Thienemanii (Lehrb. Zool., 1828: 412). 



7. The dorsal spine evidently represents a modified dermal denticle. 



