566 H. H. SWINNERTON. 



In Polypterus Traquaii* describes a large supi'a-cranial 

 fontanelle (71, p. 167) bounded laterally by the upper edges 

 of the sphenoid. Pollard (92, p. 400) confirms him, but 

 further remarks on the presence of a small sheet of cartilage 

 in the middle of the fontanelle, which he says " may be 

 regarded as indicating the former existence of a complete 

 cartilaginous tegmen cranii ; " and in a later paper (95, p. 414) 

 he speaks of it as the homologue of the epiphysial cartilage, 

 and of the space in front as homologous with Gegenbaur's 

 "Prsefontalliicke.'^ The true supra-cranial fontanelle, how- 

 ever, is the whole region enclosed by the supra-orbital band, 

 and is much larger than the one described by Traquair under 

 that name. The so-called sphenoid has not the same place 

 relations as the alisphenoid and orbito-sphenoid, for in Tele- 

 osts these bones are continuous above with the supra-orbital 

 band ; in Polypterus, however, its upper border lies some dis- 

 tance inside of, and is separated by a wide space from this. 

 Strictly speaking, therefore, the region between the trabeculae 

 and the supra-orbital band is unoccupied either by cartilage or 

 bone, and in any case the existence of a wide space between it 

 and the sphenoid betokens the existence of a large, morpho- 

 logically lateral, fontanelle. The auditory capsule has no 

 internal wall, and the cranial floor possesses an exceptionally 

 well-developed pituitary fontanelle. 



Among cartilaginous Ganoids the cranium of the adult 

 sturgeon is so specialised in respect to the growth of carti- 

 lage that it is natural neither to expect nor to find any fouta- 

 nelles. Nevertheless these do exist during development. 

 The most instructive stage in this connection is that described 

 by Parker (82) as Stage III. In this the cranium possesses 

 a large fontanelle in the roof from end to end. The supra- 

 orbital bands project towards each other in the region lying- 

 in front of the mid-brain, and thus indicate an epiphysial bar. 

 In the side walls the "optic foramen is a fenestra" (158), and 

 there seems to be a separate fenestra for the exit of the fifth 

 nerve also. The pituitary fontanelle, though small, is 

 present, and the auditory capsule lacks a cranial wall. 



