AN ICHTHYOSAURUS FEOM THE LIAS OF WHITBY. 639 



The bone, as usual, is slender on the lateral alveolar border in front 

 and slender behind. It extends below the lachrymal bone, and 

 joins the malar at about the anterior border of the orbit; and thus 

 excludes the lachrymal bone from the base of the skull. The 

 lachrymal hones (I) extend between the orbits and nares ; they are 

 broad hatehet-shaped bones. The inferior border is convex, and 

 the superior border sinuous. The posterior border is concave, and 

 the bone has a spine-like prolongation extending along the anterior 

 part of the base of the orbit. Its least antero-posterior length 

 between the two lateral cavities into which it enters is 2| inches, 

 while its depth is 4J inches. It unites above with the nasal bone (n) 

 in front and the prefrontal bone (pf), and below with the malar bone 

 (ma) behind and the maxillary (m) in front. The surface of the 

 lachrymal bone below the nasal is somewhat impressed, as though 

 its surface might have lodged a gland ; and its margin, which enters 

 into the orbit, is rounded, thickened, elevated, and continuous with 

 that of the prefrontal bone above and the malar bone behind. The 

 large size of the lachrymal bone and its great antero-posterior ex- 

 tent between the orbit and narine make one of the most distinctive 

 characters of the species. 



The orbit (0) is large and oval, much less vertical than usual, since 

 the transverse measurement between the base of the orbits behind 

 is somewhat less than 14 inches, while the measurement between 

 the superior borders in front is about 7| inches. Lines drawn 

 through the middle of the orbits horizontally would converge some 

 distance behind the anterior termination of the nasal bones. The 

 length of the orbit is 8 inches, its depth 6| inches. It forms 

 almost the entire height of the skull, which in this position, measured 

 vertically, is less than 7 inches. Its border is rounded from within 

 outward, and the outline of the cavity is regular. It is margined 

 superiorly by the prefrontal and postfrontal bones ; the middle of 

 the base is formed of the malar bone, which is encroached upon 

 anteriorly by the lachrymal bone, which forms its anterior end, and 

 posteriorly the postorbital bone forms its posterior end. 



The malar bone {ma) is a slender bar, rounded above, and some- 

 what flattened on the underside in front, but more compressed and 

 rounded behind. Its length is about 8^ inches. It is comparatively 

 straight, though slightly curved in length and convex on the outer 

 side. Its anterior end tapers, and underlaps the lachrymal bone. 

 Its posterior third receives the postorbital bone above (po), and its 

 posterior extremity is more compressed from below upward for 

 a length of an inch and a quarter, and is received between the post- 

 orbital and quadrato-jugal bones {qj). 



The postorbital bone{po)is somewhat broader than usual, and hence 

 has not quite so much of a crescent shape, its outline being more 

 that of a boot, the anterior concavity forming the orbital border. 

 The length of the base which rests on the malar is about 3|- inches, 

 while the width of the bone in its upper part is less than half as 

 much. The base is comparatively straight, as is the posterior 

 border, which joins the quadrato-jugal bone (qj), and in its upper 



