GANOIDEI. 4 1 7 



Sturgeon attain a great size, one the Beluga often measur- 

 ing twelve or fifteen feet in length. They are commercially of 

 considerable importance, the swimming-bladder yielding most 

 of the isinglass of commerce, whilst the roe is largely employed 

 as a delicacy under the name of caviare. 



Two or three fossil forms belonging to the Sturionida are all 

 that are at present known ; and by far the greater number of 

 extinct Placoganoids belong to the family Ostracostei, estab- 

 lished by Owen, and characterised by the fact that the head, 

 and generally the anterior part of the trunk as well, was encased 

 in a strong armour composed of numerous large ganoid plates, 

 immovably joined to one another. The posterior extremity of 

 the body was more or less completely unprotected, and, whilst 

 the notochord was persistent, the peripheral elements of the 

 vertebrae namely, the neural and haemal spines were ossified. 

 The following are the more remarkable forms belonging to this 

 section : 



a. Pterichthys. This is one of the most singular of fossil 

 fishes, and was first discovered in the Old Red Sandstone by 

 the late Hugh Miller. The whole of the head and the anterior 

 part of the trunk were defended by a buckler of large ganoid 

 plates, those covering the trunk forming a back-plate and a 

 breast-plate, articulated together at the sides. 



The rest of the body was covered with small ganoid scales 

 (fig. 177). A small dorsal fin, a pair of ventrals, a pair of pec- 

 torals, and a heterocercal tail-fin were present. The form of 

 the pectoral fins is the peculiar characteristic of the Pterichthys. 

 These were in the form of two long curved spines, something 

 like wings, covered by finely-tuberculated ganoid plates. From 

 their form, they cannot have been of much use in swimming ; 



Fig. 176. Cepkalaspis Lyellii. 



but they probably, as suggested by Owen, enabled the fish to 

 shuffle along the sandy bottom of the sea, if left dry at low 

 water. 



2 D 



