110 REPORT 1839. 



their locomotive extremities than any other species of the genus. 

 They are most easily and certainly distinguished from each other 

 by the form and relative size of the head and teeth. 



Localities. — The Ich. communis is the most common species 

 in the lias at Lyme and Charmouth. It also occurs in the lias 

 limestone at Street^ but is much rarer there than the Ich. inter- 

 medius. Remains of the Ich, communis liave been met with in 

 the lias at Keynsham, and near Nembroth, Bristol. A beautiful 

 head and other parts less complete of the Ich. comtmmis, from 

 Barrow-on-Soar, are preserved in the museum of the Philoso- 

 phical Institution of Birmingham. This species is associated in 

 the lias at Barrow with the Ich. tenuirostris and interme- 

 dins. 



Prof. Sedgwick has parts of a specimen of this species from 

 the lias of Stratford-on-Avon. Two skeletons of young Ich. 

 communis in the Professor's collection both present the usual 

 abrupt bend in the tail. 



The Ich. communis is undoubtedly present in the lias at Boll 

 in Wirtemberg, where it is associated principally with the Ich. 

 tenuirostris. 



Ichthyosaurus intermedins. 



Mr. Conybeare thus characterizes this species : ^^ The upper 

 part of the teeth is much more acutely conical than in the Ich. 

 communis, and the striae less prominent, yet they are less slen- 

 der than in /. tenuirostris','' and whereas in /. com?nunis ?ind I. 

 Platyodon the coronoid (surangular) disappears on the out- 

 side, (being overlaid and concealed by the overhanging flap of 

 the dental,) before the similar concealment of the angular bone, 

 in Ichthyosaurus intermedius the angular draws itself up be- 

 neath the coronoid before the coronoid is thus covered up itself*. 



The lower surface of the basi-occipital bone is but slightly 

 excavated anterior to the condyle. The maxillary portion of 

 the skull is relatively shorter, and converges more regularly to 

 the snout, than in the Ich. cotmnunis; and the teeth are longer, 

 more slender, and more numerous. 



In a skeleton in Mr. Johnson's museum at Bristol I counted 

 :H: teeth. 



The vertebrae present simple concavities at their anterior and 

 posterior extremities ; they increase in general size, and their 

 spines grow wider in the antero-posterior diameter from the 

 cervical to the pelvic region, and thence gradually diminish. 



* Mr. Conybeare refers to the figure of a beautiful specimen (PL XVII. 

 p. 112. vol. i., 2nd Series, Geol. Trans.) as displaying the latter structure. It 

 is nevertheless obvious in that figure that the so called coronoid x x disappears 

 beneath the dental a before the angular piece v similarly disappears. 



