68 



PISCES. 



than a cranial roof. Homosteus milleri, from the Old Red Sandstone of 

 Caithness and Orkney, has a shield measuring 0-3 m. across ; at least 

 one species from Dorpat, Livonia, is still larger. 



FIG. 53. 



Homosteus milleri ; cranial and dorsal shield, one-sixth nat. size. L. Old Ked 

 Sandstone ; Caithness. A, B, C, undetermined bones ; a.d.l., anterior 

 dorso-lateral ; a.e. , ethmoid ; c, central ; e.o. , external occipital ; m, mar- 

 ginal; m.d., median dorsal; m.o. , median occipital; o, orbit; p.d.L, 

 posterior dorso-lateral; p.o., preorbital ; pt.e., pineal; pt.o., postorbital. 

 The double lines indicate the course of the mucus-canals. (After Traquair.) 



Dinichthys. A genus comprising fishes of gigantic size, the head- 

 shield sometimes measuring a metre across. The elements of the head- 

 shield are almost or completely fused together in the adult, and the 

 pineal plate exhibits a minute perforation above the pineal body. The 

 orbits form notches in the shield, and the eyes are provided with a ring 

 of a few sclerotic plates. The toothless maxillo-suborbital is distinct. 

 The mandibular rami are suturally united at the symphysis, each beak- 

 shaped in front and bearing a short, single series of acute teeth anchylosed 

 with its border. The anterior upper teeth are also beak-shaped. The 

 armour of the trunk closely resembles that of Coccosteus and Homosteus. 

 The axial skeleton and pelvic fins also seem to have been similar to those 

 of Coccosteus. The typical species (Dinichthys hertzeri) and several others 



