50 BRITISH FOSSIL REPTILES. 



lateral production of the medial piece. A second styliform haemapophysis (h) is similarly 

 adapted to the fore part of the foregoing style ; but as it approaches its pleurapophysis 

 it is slightly thickened, and is joined by a truncate end to that of the pleurapophysis (pi). 

 Of the five bones which thus constitute the haemapophysial part of the thoracic-abdominal 

 hsemal arch, the two on each side I regard as a divided haemapophysis, and the trans- 

 versely extended medial piece as the haemal spine, or abdominal " sterneber.'' A small 

 but strong triradiate haemal spine (" episternum ") closes the dislocated occipital haemal 

 arch formed by the modified pleurapophyses (" scapulae '') and hajiuapophyses 

 ("clavicles"). 



In the caudal region the centrum, save at the terminal pinnigerous part, shows a small 

 tubercle at or near each of the four angles of the quadrilateral space (Tab. XVIII, figs. 9, 

 12), forming the lower surface, the anterior tubercles (hy, hy) being the largest. Each has 

 an articular surface, and the contiguous ones of coadapted centrums give attachment to 

 the base of a short and slightly bent haemapophysis. These elements, of the same pair, 

 converge as they descend, but do not coalesce to form a " chevron-bone," nor is a haemal 

 spine developed. These inferior or haemal arches cease on the terminal twenty or more 

 vertebrae ; in most of these the centrum is subcompressed, especially where the seeming 

 fracture or abrupt bend (Tab. XXII, fig. 8) takes place. 



b. Bones of the Head. — The resiilts of my studies in the craniology of the species of 

 Ichthyosaurus, subsequent to the ' Report on British Eossil Reptiles ' of 1839, were given 

 in the ' Hunterian Lectures,' at the Royal College of Surgeons, London, 1855; and in 

 those delivered at the jMuseum of Practical Geology, Jermyn Street, in 1858.^ I have 

 found but little to add or alter in the course of subsequent researches in the preparation 

 of the present Chapter. 



The general form of the skull of the typical species. Ichthyosaurus communis 

 (Tab. XXIV, fig. 1), as in Ich. breviceps, Ich. intermedius, Ich. lonchiodon, and Ich. 

 platyodon^ resembles that of the Cetaceous Dolphins {Delphinus tursio and Delphinus 

 delphis). In Ich. acutirostris (ib., fig. 2) the beak is produced to the shape of that of a 

 gigantic stork, while Ichthyosaurus tenuirostris and Ich. longirostris (ib., fig. 3) rival or 

 surpass the Delphinus [IniaJ yanyeticus in the length and slenderuess of the jaws. 



The main difference in the Sea-reptiles lies in the restricted capacity of the brain-case, 

 the seeming expanse of the cranium being due to the great depth and breadth of the post- 

 orbital part of the zygomatic arches or outer walls of the temporal fossae. A more essential 

 departure from the warm-blooded Vertebrates is the persistent individuality of those 

 cranial elements which, though primitively distinct, become blended into single bones in 



1 Notes of these Lectures were published iu the ' Annals and Magazine of Natural History,' 3rd 

 series, vol. i, 185S, p. 388 et seq. 



2 See Clift's excellent figure in ' Philos. Trans.,' mdcccxiv, pi. xvii. 



