526 



FISHES 



small polygonal plates. All* the plates are superficially orna- 

 mented by small stellate tubercles. The ventral armature (Fig. 

 315) is similar to the dorsal. A large mental plate forms the 

 hinder marsiu of the transverse slit-like mouth, the anterior 

 limit of which is defined by the rostral plates already mentioned. 

 Laterally may be seen a pair of small plates (x), each perforated 



by a small aperture, and prob- 

 ably indicating the position 

 of some kind of sense-organ. 

 Posteriorly there is a large 

 median ventral plate, in rela- 

 tion with a pair of anterior 

 and a pair of posterior ven- 

 tral plates. The areas between 

 the larger plates are filled in 

 by numerous small polygonal 

 plates. It is possible that 

 there is a single external 

 branchial aperture on each 

 jP' ' side, near the postero-lateral 

 angle of the shield and be- 

 hind the posterior ventro- 

 lateral plate. The sole repre- 

 sentative of the family is 

 Drepanaspis gemundenensis, 

 from the Lower Devonian of 

 Gemiinden in Ehenish Prussia. 

 Large examples of this fossil 

 must have exceeded two feet 

 in length. 



Fam. 3. Psammosteidae. 

 — To this family are referred 



Fia. 315.— Ventral surface of Drepanaspis certain dermal plateS OCCUr- 

 (tail ill profile). a.v.L Anterior ventre- . . i r 



lateral plates ; e.I, external lateral ; m, ^ng, m a more Or leSS frag- 



mentai plate ; m.v, mid-ventral ; p.i, pos- mentarv Condition, in the Old 



tero-lateral ; _/J.i'.?, posterior ventro-lateral ; _ _ i -r\ 



r, rostral ; x, orbit or sensory plate. The P^ed Sandstone and Devonian 



mouth andthe supposed cloacal aperture are formations of Great Britain 

 indicated in black. (From Traquair.) 



and Eussia. In their size 

 and shape, and in their stellate tubercles, these have been com- 

 pared to the dorsal, postero - lateral, and ventral plates of 



