DIPNOI. 269 



arthrodira, they are few. In the existing species parasphenoid, 

 vomers, palatoquadrate and squamosal, as well as dentary, angu- 

 lar and opercular in the lower jaw, are more or less certainly 

 to be recognized ; but beside these there are several bones in the 

 cranial roof which are not to be homologized with those in other 

 groups. 



The operculum is supported by bones (operculum, inter- 

 operculum), and the hyoid and (four or five) branchial arches are 

 cartilaginous. The pectoral and pelvic arches are cartilaginous, 

 the former with membrane bones of the fish-type (p. 174) weakly 

 developed. The pelvis consists of a median plate with (in recent 

 forms) three anterior horns. The fins themselves are very prim- 

 itive, and consist of an axial portion, from which, in Ceratodus, 

 biserial cartilaginous fin rays arise. In many fossils the ante- 



FlG. 270. Restoration of Dinichthys, from Dean. 



rior part of the body is enclosed in a strong armor of bony der- 

 mal plates, there being a hinge between the dorsal plates and 

 the base of the skull. 



The brain differs from that of the teleostomes in the ner- 

 vous character of the cerebral mantle. The two hemispheres are 

 united in Ccratodus, but in Protoptcrns they are distinct back to 

 the anterior commissure. The mid brain is paired in Ccratodus, 

 simple in Protoptcrus ; the cerebellum is but a small transverse 

 fold. The pineal structures have a long stalk, while the envel- 

 opes of the brain are richly developed, and in Protopterus these 

 enter above the fossa rhomboidalis into close connection with 

 the endolymphatic system of the ear. An optic chiasma is 

 present. 



The teeth are few in number, and are apparently formed by 

 the fusion of several primitive teeth. Of these there is a pair 

 of larger grinding plates borne on the palatopterygoids, a much 



