PIIOI KKDINflS (ir THE 



not yet interpreted, perhaps related to the olfaciorv apparatus. 

 A coiispieuoiis ovoid plate behind the orbits tills a vaciiiiy in the 

 roof of the brain-case. Along each side of the head-shield there 

 extends a lono; narrow vacuity lilled with loose polygonal ])lates, 

 whicli form a llexihie roof to a chamber within the inroUed rim of 

 the shield. I regard this chamber as bianchiai, for it opcMis 

 behind just within tlie cornu (or postero-lateral angle) of the 

 head-shield, where a flexible Map stilVened by polygonal plates 

 may ho regarded as an opercidnm. In some specimens of the 

 closelv-allied genus Aucheudspis (or I'lii/tsles) there are indeed 

 traces a]>i)arentlv of the branchial arches close to the cham])er on 

 each side*. The month must have been j)laced far forwards 

 on the ventr.il surface, but no hard parts related to it have been 

 observed. The body-cavity extended far back in the scaly tail, 



Fig. 4. 



Restoration of Ccpludaspis murchisoni from the Downlonian I'ass.tge Heds of 

 irerefordshire. about one-half nat. size. The head shown from above 

 liie tail twisted to be seen mainly in side-view. 



where the remote cloacal opening has been seen. CepJiahtspis has 

 only one dorsal iin, but the median scales in front of it form a 

 curious elevated ridge along the back, and two dorsal tins have 

 been observed in the allied genera Ai-erasj>is and i\Iia-asj)is +. AW 

 the tin-membranes are scaly, without true fin-rays. 



Auchenaspis (or T}iyesies)+ is closely similar to Oej>hiihtsi>is, 

 only having some of the anterior body-scales fused with the 

 hinder border of the head-shield. Eul-eraspis is interesting as 

 showing the flexible roof of the pair of branchial chambers sub- 

 divided by cross-bars; while in Tremi(tas2ns'i} tiiis roof is reduced 



* J. V. Rohon, Yerhandl. russ. k. min. Ores. St. Petersb. [2] vol. xxxiii. 

 (189r»), p. 17; also Bull. Acad. tmp. Sci. SL-l'ttersb. vol. iv. ^1S9(>). p. --'31, 

 with (igs. 



t J. Ki.-vr, Skrift. \ icensk.-Selsk. Kristiania. Mat.-naturv. Kl. I'Jll, no. 7, 

 pj). IG, 17. 



+ J. v. Rohon. Mem. Acad. Imp. Sci. St.-lVtersb. [7J vol. xxxviii. no. 13 

 (1892), pp. 12-37, with ligs. 



§ W. Patten, M6m. Acad. Imp. Sci. St.-1'etersb. [8] vol. xiii. no. 5 (1903). 



