104 PISCES. 



in other genera. The plates covering the ethmoidal region (eth.} are very 

 irregular and extend backwards more or less between the frontals ; the 

 series is, indeed, continued backwards by another median plate which 

 meets, or is even fused with the supraoccipital. The cranial shield as a 

 whole is thus very suggestive of that of Acipenser. The cheek-plates 

 (fig. 73 B) are small, thin, and apparently without any very definite arrange- 

 ment. One plate, larger than the remainder, may be the maxilla (mx.), but 

 it is delicate and toothless. The premaxillae are deep and narrow, resting 

 upon the anterior extremity of the vomer, and covered with the ethmoidal 

 dermal plates. The hyomandibular is broadest above, tapering below; 

 the pterygo-palatine arcade is delicate and toothless and is directly fused 

 for a considerable distance with the edge of the basicranial axis. The 

 thickened articular end of the quadrate is turned forwards so that the 

 facette for the mandible is in a vertical plane. Each maudibular ramus 

 consists of the usual five elements, the splenial preponderating and enter- 

 ing the symphysis. Grinding teeth are arranged on the vomer in more or 

 less bilaterally symmetrical longitudinal series ; similar teeth also occupy 

 the enlarged splenial ; but those of the premaxilla and deutary are pre- 

 hensile. No successional teeth have been observed. The operculum is 

 small, deep, and narrow ; the preoperculum is very large, triangular, and 

 much expanded below. Suboperculum and interoperculum are wanting, 

 and there are only two branchiostegal rays. The space between the rami 

 of the mandible is covered with small polygonal plates. The neural and 

 haemal arches are well ossified, but are not expanded to encircle the noto- 

 chord. The clavicle is small, slightly expanded below ; the pelvic fin- 

 supports are unknown. The external bones and scales are ornamented 

 with tubercles of ganoine, and the ridge-scales are inconspicuous. Two 

 species are known, Mesturus verrucosus from the Lithographic Stone of 

 Bavaria, and M. leedsi from the Oxford Clay of Peterborough. 



Gryrodus. As Mesturus, but the caudal fin forked, and the scales not 

 suturally united. The longitudinal series of teeth on the vomer and 

 splenial are a little more regular than in the last genus. As additional 

 characters may be noted, (i.) the course of the "lateral line" from the 

 outer supratemporal to the middle of the caudal pedicle, with a second 

 canal along the top of the flank to the origin of the dorsal fin, and (ii.) a 

 problematical arched bone at the hinder end of the body-cavity. This 

 has much the appearance of a median internal bone ; while in some other 

 genera it has been interpreted as a great paired dermal plate. It is cer- 

 tainly not an enlarged support of the anal fin, and it has no connection 

 with the pelvic fins. Eight basal bones have been noticed in the pectoral 

 fin. Gyrodus is typically an Upper Jurassic genus, the best specimens 

 having been obtained from the Lithographic Stone of Bavaria (G. ma- 

 crophthalmus, O. hexagonus, &c.). 



The earliest known Pycnodont is Mesodon liassicus from the 

 Lower Lias of Leicestershire and Gloucestershire, and this is in 



