1 6 Memoir Sears Foundation for Marine Research 



for either the Bathylaconoidea or the Lyomeri, curiously modified groups that are in- 

 cluded in Parts 4 and 5 because they appear to stem back either to isospondylic or to 

 eel-like ancestry; for further details^ see Romer (75: 579—584). 



Identification. The sturgeons, the gars, the lyomerids, and the giganturoids are set 

 apart by external features so obvious that no one at all acquainted with fishes would 

 be likely to mistake any one of them for any other kind of fish that is known from the 

 western North Atlantic. Although the Isospondyli, and probably the Iniomi as well, 

 represent several phyletic lines of descent, with their members being correspondingly 

 varied in appearance, they are easily separated as a group from the other Orders that 

 are dealt with in Parts 3-5. 



The following Key to the Orders described in these three volumes is ofiFered solely 

 as an aid to identification. The reader is referred to the sections dealing with the several 

 Orders for information as to their internal features. 



Key to External Characters of Orders Represented in 

 the Western North Atlantic 

 and Described in Parts 3—5 



I a. Separate rayed caudal fin clearly marked off from anal fin, from dorsal fin, or from 

 both anal and dorsal. 



2 a. Caudal fin with conspicuous fleshy axis (marking rearward extension of verte- 

 bral column) bending sharply upward at base of caudal and continuing out- 

 ward, close to upper margin of fin, nearly to its tip ; upper margin of fin much 

 longer than lower margin (Fig. 3). Aclpenseroidei, Part 3, p. 24. 



2 b. Caudal fin without conspicuous fleshy axis; upper margin of fin little if any 

 longer than lower margin. 



3 a. Trunk, rearward from gill openings, completely clothed below and above 

 with thick rhomboid scales in mosaic pattern, interlocking but hardly 

 overlapping, forming a flexible armor; rear boundary of fleshy base of 

 caudal fin sloping obliquely rearward-upward (Fig. 3) ; anterior part of 

 upper margin of caudal and nearly entire length of lower margin 

 edged with a series of large "fulcral" scales, shaped as in Fig. 3. 



Lepisostei, Part 3, p. 61. 



3 b. Trunk, rearward from gill openings, not clothed in a continuous armor 



of interlocking scales; rear boundary of fleshy base of caudal either nearly 



vertical or symmetrically rounded; margins of caudal fin without fulcral 



scales. 



4 a. Point of origin of pectoral fins higher on sides than upper end of 

 gill openings; fin rays not branched. Giganturoidei, Part 4. 



