RAJU: THE GENUS MONOGNATHUS 



Table 1. — Morphometric and meristic characters of the species of the genusMonognathus . 



'The predorsal distance given by Bertin is obviously wrong as the dorsal originates on myotome 12. 



the mouth, but it does not do so in other species. 

 There is a small pectoral fin. The vertebrae are 

 weakly ossified, not distinct in radiographs, but 

 about 75 myotomes could be counted, and about 

 30-35 more are estimated on the basis of the size 

 of the myotomes. Differences of this magnitude 

 could well indicate generic separation, but I hesi- 

 tate to introduce new genera in this poorly 

 known group. 



MONOGNATHUS AHLSTROMI SP. N. 



Figures IE; 2A-E 



Holotype: SIO 63-405, eastern North Pacific, 

 34'=57.0'N, 129^19.0'W, 0-2,000 m, IKMT, 1(48.5 

 mm), 29 Mar. 1962. 



Description: Body compressed except at head, 

 very delicate, covered with loose semitransparent 

 skin. Preanal region 3.6 in total length, very deep 

 due to voluminous stomach. Tail (postanal region) 

 very narrow, 1.3 in total length, tapering 

 gradually to a point. Maximum depth before anus, 

 11.0 in total length (may vary according to quan- 

 tity of contents of stomach). Head deep, large, 

 9.0 in total length; cranium very small and weak. 

 Snout small, 5.4 in head, membranous. Olfactory 

 organ reduced, a small curved tube open at both 

 ends. Eye rudimentary, oval, vertical length 

 18.0 in head, lens round, extremely small. Gape 

 of mouth reaching far beyond eye. Region of upper 

 jaw membranous, devoid of teeth, no maxilla 



distinguishable. Ethmoid tooth short, hollow, tip 

 sharp and bifid, and does not project into 

 mouth as in M. isaacsi; it appears as though it is 

 lodged in a sheath and comes out of the sheath 

 through an opening on the membranous palate 

 only when pressed. Ethmoid glands paired, oval 

 (0.8 mm X 0.4 mm) on either side of tooth. Lower 

 jaw long, 9.7 in total length, tip armed with three 

 sharp closely packed teeth followed by series of six 

 pointed, equally spaced, triangular teeth. Oper- 

 cular bones and branchiostegals absent. Gills 

 four, covered by delicate transparent membrane. 

 Gill filaments short, foliaceous. Gill openings 

 ventrolateral, moderate in size. Pectoral fin 

 absent. Myotomes 28 + 85 = 113, W-shaped. 

 Esophagus short, followed by thickened girdle- 

 like region. Stomach large, bulging with opening 

 just posterior to anus. This opening of the stomach 

 appears to be a structural feature and not a 

 wound associated with capture. Intestine a 

 narrow straight tube on right side of body, 

 opening to exterior on right side. Liver small and 

 lobular. Dorsal fin originating on myotome 14, 

 moderately high, 90 unsegmented rays. Caudal 

 fin represented by an enlarged, paddlelike struc- 

 ture without fin rays. Vertebral centra short. 



Pigmentation: No trace of any larval mid- 

 lateral pigmentation. A few dendritic melano- 

 phores on snout and lower jaw. Dendritic mel- 

 anophores around esophageal and liver region 

 inside body and on body wall in stomach and anal 



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