142 THE ANATOMY OF VEETEBEATED ANIMALS. 



are so loosely connected with the floor of the skuU, that 

 the jaws can be protruded and retracted to a considerable 

 extent. 



In Lepidosteus, Polypterus, and Amia, the skull presents 

 not only membrane bones, but, in addition, basi-occipital, 

 ex-occipital, and pro-otic ossifications of the primordial carti- 

 lage, to which others may be added. The vomers are double, 

 as in the Amphibia (? Pohjptei'us). The apparatus of the 

 jaws has become modified in accordance with the Teleos- 

 tean type of structure. The suspensorium consists of two 

 ossifications united by a cartilaginous intermediate por- 

 tion. The upper — bi-oad, and moveably articulated with the 

 periotic capsule — is the hyomandibular ; the lower answers to 

 the sympledic of osseous fishes. The cartilaginous palato- 

 quada-ate arcade is, in part, replaced by a series of bones : 

 the palatine lies in front, and is connected with the pre- 

 frontal region of the skull; behind it, lie representatives 

 of the pterygoid, the metapterygoid, the ectopterygoid ; 

 and, most posteriorly, of the quadrate bone. The last fur- 

 nishes a condyle to the articular element of the mandible. 

 The symplectic is either loosely connected with the quad- 

 rate, as in Lepidosteus, or more closely united with it, as in 

 the other genera. 



In Lepidosteus and Amia, a strong and long membrane 

 ])one, the preopermdmn, is developed on the outer side of 

 the hyomandibular and quadrate bones, and connects them 

 stiU more firmly together. 



The maxilla is represented by a series of small separate 

 ossifications in Lepidosteus. The proximal end of the 

 mandibular cartilage ossifies, and becomes a distinct arti- 

 culare. A dentary element is added on the outer, and a 

 splenial one upon the inner side of the cartilage ; and in 

 Lepidosteus, angular, supra-angular, and coronary elements 

 are added, so that the components of the mandible are as 

 numerous as in reptiles. Lepidosteus and Amia have bran- 

 chiostegal rays, but Polypterus has none — at any rate, of the 

 ordinary kind. A single jugidar plate is developed between 

 the rami of the mandible in Amia, and there are two such 



