PECTORAL AND PELVIC GIRDLES 



239 



fins, in some of the Lower Devonian Acanthodei (e.g. Climatius), 

 may be regarded in that light. 



The Pectoral and Pelvic Girdles. The pectoral girdle is more 

 primitive in Cladoselache and Pleuracanthus than in any other 

 Elasmobranch. In the former (Fig. 145, A) it may be doubted 

 if the girdle has passed beyond the basipterygial stage, and 

 although a definite girdle is present in the latter genus (Fig. 

 250) its lateral 

 halves retain their 

 primitive distinct- 

 ness. Existing 

 Elasmobranehs, in- 

 cluding the Holo- 

 cephali, have a 

 pectoral girdle in 

 the form of a 

 dorsally incomplete 

 hoop of cartilage 

 imbedded in the 

 muscles of the 

 body-wall, close 

 behind the last 

 branchial arch (Fig. 

 141). The upper 

 or dorsal portion 

 of each half is the 

 scapula, and the 

 ventral is the cora- 

 coid. Between 



FIG. 141. The right half of the pectoral girdle and the fin 

 of an Elasmobranch (Chiloscyllium). d.r, Dermal horny 

 h'bres ; meso, niesopterygium ; meta, metapteryghun : 

 pect, pectoral girdle ; pro, propterygium. (From Parker 

 and Hasvvell.) 



these two portions 



of the girdle, and defining their limits, there are articular surfaces 



for the basal cartilages of the pectoral fin. 



Cladoselache (Fig. 145, B) had no pelvic girdle, nor does it 

 appear that this primitive Elasmobranch had acquired even 

 a basipterygium. Pleuracanthus, on the contrary, had a pair of 

 pelvic rudiments distinct from well-developed basipterygia. In 

 other Elasmobranchs there is a distinct girdle, formed by the 

 median union of primitively distinct lateral rudiments, consisting 

 of a simple transverse bar of cartilage, imbedded in the ventral 

 abdominal wall, just in front of the cloacal aperture, and having 



