184 ZOOLOGY SECT. 



depends a large cutaneous flap (B, tc) abundantly supplied with 

 nerves and evidently serving as an important tactile organ. 



A still more important difference from Elasmobranchs is the 

 possession of only a single external branchial aperture (br. ap.), 

 owing to the fact that a fold of skin, the operculum (op.), extends 

 backwards from the region of the hyoid arch and covers the true 

 gill-slits, which thus come to open into a common chamber situated 

 beneath the operculum and communicating with the exterior by 

 a single secondary branchial aperture placed just anterior to the 

 shoulder-girdle : there is no spiracle. Equally characteristic is 

 the circumstance that the urinogenital aperture is distinct from and 

 behind the anus, there being no cloaca. 



There are two large dorsal fins (d.f. 1, d.f. 2) and a small ventral 

 (v. /.) ; the caudal fin (c. /.) is of the ordinary heterocercal type 

 in the adult Callorhynchus, but in the young (Fig. 864) the 

 extremity of the tail proper is not upturned, and the fin-rays are 

 arranged symmetrically above and below it, producing the form 

 of tail-fin called diphycercal. In Chimaera the tail may be produced 

 into a long whip-like filament (c.f.). The pectoral (pct.f.) and 

 pelvic (pv.f.) fins are both large, especially the former. 



In the male there is a horizontal slit (B, a. cl f .) situated a little 

 in front of the pelvic fins ; it leads into a shallow glandular pouch, 

 from which can be protruded a peculiar and indeed unique 

 apparatus, the anterior clasper (A, a. cl.), consisting of a plate 

 covered with recurved dermal teeth, to which is added, in Callo- 

 rhynchus, a plate rolled upon itself to form an incomplete tube. 

 The use of this apparatus is not known. A rudiment of the pouch 

 occurs in the female, although the clasper itself is absent. The 

 male possesses, in addition, a pair of the ordinary pterygopodia or 

 posterior claspers (ptg.), and is further distinguished by the 

 presence of a little knocker-like structure, the frontal clasper (fr. cl.), 

 on the dorsal surface of the head. In Harriotta the paired claspers 

 are poorly developed, and the frontal clasper is absent. 



The lateral line (1. I.) is an open groove in Chimsera, a closed 

 tube in Callorhynchus, and there are numerous sensory pits, arranged 

 in curved lines, on the head. The skin is smooth and silvery, and 

 bears for the most part no exoskeletal structures. There are, 

 however, delicate, recurved dermal teeth on the anterior and frontal 

 claspers, and the first dorsal fin is supported by an immense bony 

 spine or dermal defence (sp.). In the young, moreover, there is 

 a double row of small dermal teeth along the back. 



Endoskeleton. The vertebral column consists of a persistent 

 notochord with cartilaginous arches. In Chimsera, but not in 

 Callorhynchus, there are calcified rings (Fig. 859, c. r.) embedded 

 in the sheath of the notochord. The anterior neural arches are 

 fused to form a high, compressed, vertical plate, to which the first 

 dorsal fin is articulated. The cranium (Figs. 860 and 861) has a 



