POLYPTERINI 



297 



and dorsal fins each lepidotrich is related to one radial. The 

 radials supporting the dorsal finlets are each formed of a single 

 piece, which, as in the Holostei (p. 322), projects not at all beyond 

 the body into the base of the fin (Bridge [56]). 



There are four branchial slits, and neither mandibular nor 

 hyoidean gill. The gill-lamellae project freely at their end into the 

 gill-chamber ; but there is more septum preserved than in the 

 higher Actinopterygii (p. 95 and Fig. 57). A double series of 

 cartilaginous rays supports the lamellae. A remarkable pinnate 

 external gill is developed 

 on the hyoid arch of the 

 larva (Fig. 272). The ven- 

 tral air-bladder, with cellular 

 walls, and a symmetrical 

 arterial blood -supply from 

 the last branchial arch, 

 has already been described 

 (p. 223). The teeth are of 

 simple conical shape, without 

 foldings. There is a gastric 

 caecum, and the intestine 

 bears a single pyloric caecum 

 (Fig. 270). 



The brain of Polypterus 

 shows many generalised char- 

 acters (Bing and Burckhardt 

 [73], Kerr [263]). The 

 medulla is primitive in its 

 form, the thinness of its walls, 

 the large size of its ventricle, 

 and the structure of its roof. 

 The small cerebellum has FlG - 



a thin median ZOne, with Ventral view of the 'pelvic girdle and fins of 



,1.1 , .j Polypterus bichir, Geoft'r. The skeleton of the right 



thickenings at the Sides, pro- n n is completely exposed, c, anterior cartilage ; dr, 



f nrwarda Vplnw rprvp lepidotrichia supporting web of tin ; p, pelvic bone ; 



lOrwaiQ 10W, n ,,r.r, preaxial radials ; s, scale-covered lobe. (From 



pr.i 



. Joum. jticr. 



senting the valvula so much 

 developed in the higher 



Actinopterygii (p. 305). The optic lobes are of moderate size and 

 paired. The roof of the fore -brain is epithelial and crossed 

 by a deep velum transversum ; large corpora striata are de- 

 veloped below. There are no cerebral hemispheres. In the 

 young P. palrtms the olfactory bulbs are sessile ; but in the adult 

 P. bickir they are stalked. On the whole, the brain of the 

 Polypterini represents a primitive low stage of differentiation 

 leading towards the Actinopterygian type (Fig. 271). With the 

 brain of the Dipnoi or Amphibia it shows no particular affinity, 



