92 DOSSIL REPTILTA OF THE 



are comparatively slender, commonly subcylindrical ; the longer ones, or those of the trunk, 

 are longitudinally grooved on their outer surface (PI. XXI, fig. 7), as if each rib consisted 

 of two confluent more slender ones. 1 This structure is not common to all the species. 

 A transverse section which I made exposed a small central cavity. 



The haemal or costal arch is complete along the major part of the trunk. Here the 

 haemapophyses are each in two partially overlapping pieces (ib., fig. 2, h, h') ; they are more 

 slender than the pleurapophyses. 2 The median piece (haemal spine, /«?) is transversely 

 extended, symmetrical, slightly produced at its thickest midpart, forward and backward, 

 but more extended laterally, there becoming slender and diminishing to a point. A 

 similar slender piece, pointed at both ends (h'), is spliced as it were, to the fore part of each 

 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 

 haemal 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 haemapophyses 

 ("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 (PI. XXII, 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, con- 

 verge 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 (PI. XXVI, fig. 8) takes place. 



b. Bones of the Head. — The results 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 Museum of Practical Geology, Jermyn Street, in 1858. 3 I have 

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

 of the present Monograph. 



1 Clift, 'Phil. Trans.,' mdcccxiv, pi. xix (1814, Ichthyosaurus platyodon). 



2 lb., 'Phil. Trans.,' mdcccxix, pi. xiv (1819, Ichthyosaurus communis). 



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

 series, vol. i, 1858, p. 388 et seq. 



