LIASSIC ICHTHYOSAURS. 69 



that of the skull, and the length of the ' snout,' or upper jaw, anterior to the orbit, is 

 three and a quarter times that of the orbit 



Of great breadth posteriorly, the skull narrows to the fore part of the orbits ; thence 

 the upper jaw contracts rapidly, afterwards gradually, to the anterior almost pointed 

 end. As it advances the upper jaw becomes subcorapressed. In profile, after the 

 concavity due to the sinking of the cranium anterior to the orbit, the line goes straight 

 to near the end of the upper jaw, where it rapidly sinks to the alveolar border. 



The chief characters of the present species are afforded by the teeth and the pectoral 

 paddles. 



The teeth (Tab. XX, figs. 5, 5') are more numerous and smaller than in Ich. brevi- 

 ceps, but, in comparison with the majority of the known species, are proportionately large. 

 They have an expanded or ventricose root, contracting to a conical, slightly aduncate 

 crown, with a subcircular transverse section. The apex is subacute, but there is no 

 coronal trenchant margin ; the enamel is impressed by fine longitudinal grooves, with 

 intervening ridges. These finer ridges are somewhat abruptly divided from the coarser 

 ones of the root by a smooth tract marking the base of the enamelled crown. Viewed in 

 the series the teeth seem to taper less regularly, often more quickly, to the apex than in 

 other species. The upper jaw bears on each side from forty to fifty teeth, of which 

 sixteen or eighteen may be implanted in the maxillary bone, the rest in the premaxillary. 

 Each ramus of the mandible may support a few teeth more than those opposed to them 

 in the upper jaw. 



The midfrontals take the chief share in the formation of the parietal foramen, and 

 are excluded from the upper border of the orbit by the pre- and postfrontals. The 

 sclerotic plates are sixteen or seventeen in number. 



The nostril is bounded by a straight line above, contributed by the nasal bone, and 

 by a curved line below, due to the lacrymal and premaxillary. The maxillary is 

 excluded from that opening ; it receives the pointed fore end of the malar in a notch ; that 

 slender bone forms the whole of the inferior border of the orbit. 



In the mandible the surangular extends forward beyond the angular. The dentary 

 elements unite to form the major part of the symphysis, a small hind portion being 

 contributed by the splenials (Tab. XXVI, fig. 4, 32')- The symphysis so constituted had, 

 in a lower jaw 2 feet 9 inches in length, an extent of 9 inches. 



The vertebrae are shorter, or their centrums have a minor antero-posterior extent in 

 proportion to their breadth, than in some other species (/c7<. intermedius, Ich. latimanus, 

 e._^.),but are longer than inlch. hrachyspondylus. I have counted forty vertebrae between 

 the occiput and the " sacrum," conventionally so calling the centrum nearest the iliac 

 bones. From this to the end of the tail follow, at least, one hundred vertebrae, of which 

 seventy-five no longer support pleurapophyses. The total number of vertebrae in Ichthyo- 

 saurus communis may be set down at 140. 



The episternum shows no marked specific character. The coracoids have the anterior 



