1859.] EGERTOX — OLD RED FISHES, 127 



the latter to belong to Asterolepis, with the remains of which genus 

 it is associated. Platygnathus is nearly allied to Holoptychius, but 

 diners from that genus in the extraordinary development of the 

 single fins. 



Family Placodermata*. — This family- definition was first pro- 

 posed by Prof. M'Coy, for the reception of all the genera included 

 in the Cepludaspidce, Agass., except Cephalaspis, together with 

 some other genera assigned by Agassiz to the Ccelacanihi. Prof. 

 Pander has adopted this family-term, and includes in it the follow- 

 ing five genera : Pterichthys, Agass. (Asterolepis, Eichw.) ; Coccosteus, 

 Agass. ; Asterolepis, Agass. (Homosteus, Asmus); Heterosteus, Asmus ; 

 and Chelyophorus, Agass. The distinction (as I understand it) be- 

 tween this family and the Cephalaspidos is, that whereas in the latter 

 the head only is encased, in the former the thorax is also invested 

 with bony plates. Cephalaspis, Pteraspis, and Auchenaspis would 

 consequently constitute the Cephalaspid family, — Pterichthys and Coc- 

 costeus being the types of the Placoderms. Chelyophorus is probably 

 a member of the same family. "Whether Asterolepis and Heterosteus 

 belong to it must depend upon further investigation. Hugh Miller 

 describes the plates of Asterolepis as the homologues of true cranial 

 bones, and assigns to this fish the scales figured at page 72 of the 

 ' Footprints.' Pander, on the contrary, maintains that the small 

 anterior plates only are true cranial plates, and that the larger plates 

 are homologous to the thoracic plates of Coccosteus. The plate figured 

 by Hugh Miller, at page 80. as a hyoid plate, Pander assumes to be 

 the posterior dorsal plate, homologous to the large cuspidate plate of 

 Coccosteus. It is clear, then, that the family affinity of this genus 

 must depend upon the solution of this moot point. I regret I have 

 no materials to throw light on this subject. 



Ptekk iitiivs. — In the memoir on Pterichthys read before the Geo- 

 logical Society in 1848, 1 described two plates as the posterior ventral 

 plates. Prof. M'Coy is quite correct in considering these as the 

 prolongations of the posterior ventro-lateral plates. The transverse 

 depression frequently causes a fracture in thai direction, which 1 

 had erroneously considered a suture. The tin which 1 described as a 

 dorsal 6s is truly such, and not an anal tin, as supposed by Prof. 

 Bf'Coy. 1 have a specimen of Pterichthys quadratics in profile, in 

 which the tin is preserved in situ,. Pterichthys had also two ventral 

 tin-, a- conjectured by the late Lady Gordon Cumming. They are 

 supported by Bpines, and are identical in form and size with the 

 dorsal tin. I have no evidence of an anal tin. 



Coccosteus. In detailing the generic characters of Coccosteus, 



Prof. M'Coy has made a grave mistake (which led him into siil.se- 



quenl error) in ft«rigning five plates as the complement of the sub- 

 thoracic disc. This is the correct number in the genus Pterichthys; 



but in CoCCOSteuS there are tWO median plates, one central and 



lozenge-shaped, the other anterior and Bubtriangular, wedged in 

 between the two anterior ventral plates, — m airing the total Dumber 



six+. 



■ Prof, Owen object* to thu term, ^ being synonymous with I 

 Bee Supplement 



