CERATODUS. 19 



a dental plate, and the angular giving off a coronoid process. Branchiostegals unos- 

 sified, gular plates none. Teeth. — A pair of vomerine cutting teeth ; palatal and 

 mandibular dental plates, ankylosed to bone. Dental plates composed of dentine, rising 

 into folds or ridges externally, anterior end broader and more elevated, inner border 

 smooth ; punctate, minute structure tubular ; pulp-cavity persisting in the centre of each 

 plate until the individual is nearly adult. Vertebral column notochordal ; neural and 

 haemal arches ossified except at the ends ; bearing neural spines, inter-neurals, and in 

 the vertical fin a double row of cartilaginous dermo-neurals. About twenty-seven pairs 

 of ribs. The haemapophyses united in the caudal region, and bearing haemal, inter- 

 haemal, and a double row of cartilaginous dermo-haemal spines. Pectoral arch. — A 

 supra-scapula attached to skull, and forming the posterior wall of the gill-chamber; 

 clavicle large, partly cartilaginous, the bony investment divided by a transverse suture. 

 Pelvic arch. — -A single pelvic cartilage giving off two processes on either side, of which 

 the lower supports the limb. Pectoral fin. — Acute lobate, a broad fringe of fin-rays 

 on the pre-axial and post-axial borders. Ventral fin similar j abdominal. Vertical fin 

 continuous from the middle of the back to the vent, diphy cereal. Scales large, cycloid, 

 faintly sculptured, calcified externally. Lateral line extending from head to tail, nearly 

 central. Heart. — Bulbus arteriosus imperfectly two-chambered, with one or two trans- 

 verse rows of perfect valves, and three or two rows of imperfect valves behind ; an 

 incomplete ventricular septum ; no auricular septum. Branchial arches five, of which 

 four bear gills ; the gills attached beyond the branchial arches to the outer wall of the 

 gill-chamber. No mandibular pseudo-branchia ; no spiracle ; opercular gill (not 

 functional). Lv,ng single, cellular, with duct and glottis. Intestine with spiral valve. 

 Ovaries lamellated, unconnected with oviducts. Abdominal pores two, behind the vent. 

 Oviducts and vasa deferentia unconnected with ureters. Optic nerves not decussating. 



References. — Agassiz, ' Poisson Fossiles,' vol. iii; Giinther, ' Phil. Trans.,' vol. 161, 

 park 2 (1872) ; Krefft, < Proc. Zool. Sec.,' 1870, p. 221 ; Huxley, ' Proc. Zool. Soc.,' 1876, 

 p. 24. The muscles are described in Humphry's ' Observations in Myology,' p. 91. 



The genus Ceratodus was first described by Agassiz l from scanty materials derived 

 from Aust Cliff on the Severn, from the Trias of Germany, and from the Oolite of 

 Stonesfield. Only the teeth were known, and of these nearly every example seemed to 

 offer features peculiar to itself. Out of seventeen specimens known to him, Agassiz 

 selected fourteen as types of possibly distinct species. He decided from the form of the 

 teeth that they must have been set in opposed pairs, one on each side of the mouth, that 

 the denticles were external, and the narrow end foremost. The substance of the tooth 

 he found to consist of tubular " enamel " (dentine) investing a bony base. The zoo- 

 logical position of Ceratodus was hard to determine. Agassiz was at first disposed to 

 place it near Psammodus, but when he found reason to attribute to the new genus not 

 several rows of teeth, as in Psammodus, but one pair only in each jaw, be inclined to 

 1 ' Poissons Fossile*,' torn, iii, p. 129 et seq. (1833-43). 



