64 Memoir Sears Foundation for Marine Research 



latter a rearward extension of the basal cartilaginous element of the fin. Development 

 either oviparous/ ovoviviparous,' or viviparous;* embryos with transitory external gills.' 

 For convenience, the modern representatives of this subclass may be grouped in two 

 orders; the one to include all living sharks as well as the fossil group (hybodonts) from 

 which they appear to have descended, the other to include the skates and rays, which have 

 probably descended from the hybodonts also. 



Key to Orders of Modern Elasmobranchs 



I a. Gill openings at least partly lateral ; edges of pectoral fins not attached to sides of head 

 anterior to gill openings; upper margin of orbit free from eyeball (eyelid free). 



Selachii; Sharks, p. 64. 



lb. Gill openings confined to ventral surface; edges of pectoral fins attached to sides of 

 head anterior to gill openings; upper margin of orbit not free from eyeball (no free 

 eyelid). Batoidei; Skates, Rays.* 



Order SELACHII 

 Modern Sharks 



Characters. Gill openings at least partly lateral; edges of pectoral fins not attached 

 to sides of head anterior to gill openings; upper edges of orbits free from eyeballs, as free 

 eyelids. Other than as indicated above, no sharp lines can be drawn between the sharks on 

 the one hand and the skates and rays on the other, so far as external characters are con- 

 cerned; the gap between the prevalent cylindrical body shape of the former and the much 

 flattened form of the latter is bridged by one group of true sharks (Squatinoidea, p. 533). 



Skeletal differences between the two groups are considerable, however, correspond- 

 ing chiefly to the highly specialized external features of the rays. Thus, to mention only 

 the most obvious, among sharks the propterygial cartilage of the pectoral bears many 

 fewer radials than the metapterygial and is smaller than the latter,^ while the reverse is 

 the case among rays. Among sharks the shoulder girdle is neither directly nor firmly at- 

 tached to the vertebral column, nor are its elements united above, while in rays it is at- 

 tached above by a separate scapular element, or elements. The suspension of the jaws 

 differs also between the two groups; in sharks the ceratohyal cartilage is attached to the 

 lower end of the hyomandibular as well as more or less intimately to the posterior end of 

 the lower jaw (Meckel's cartilage), thus assisting to support the latter; in rays (typically, 



2. Eggs laid before hatcliing. 



3. Eggs hatching and embryos developing within the mother, but without placental attachment. 



4. Embryos attached to the uterine wall of the mother by a yolk-sac placenta. 



5. For an excellent and comprehensive account of the morphology of the elasmobranchs, see Daniel (Elasmobranch 

 Fishes, Univ. Calif. Press, 1934). 



6. The skates and rays are classed as a suborder only by some authors. But the skeletal differences between them and 

 the typical sharks discussed above (p. 64) seem to us sufficient to set them apart as a separate order. 



1. This applies even to the Squatinoidea (pp. 77, 533). 



