298 PTlor. U. G. SEELET ON SIMILITUDES OF 



process replaced by tubercles for the rib, in the Bot dissimilar 

 neural arch being permanently separated from the centrum, while 

 chevron bones are wanting in the tail, the caudal vertebrae elon- 

 gate towards its end, and the prepelvic ribs have a double articu- 

 lation with every vertebra. 



No mammal has the rib articulated to the centrum by two arti- 

 cular facets ; nor have mammals caudal ribs, as in Ichthyosaurus ; 

 nor are the short sternal bones ever transversely elongated to form 

 median pieces which lap along the sides of sternal ribs. 



The resemblance of the ichthyosaurian pectoral girdle to that 

 of monotreme mammals is not close ; for in the monotreme 

 the coracoids are divided so as to form a pair of precoracoids 

 which meet mesially, and are overlapped by the interclavicle, 

 while no such division is seen in Ichthyosaurs ; the scapula is a 

 squamous broadly expanded bone with an acromion, very unlike 

 the narrow elongated bone o^ Ichthyosaurus; the clavicles of the 

 monotreme only extend to the acromion, instead of lapping along 

 the whole anterior margin of the scapula as in Ichthyosaurus ; 

 and the interclavicle laps behind the clavicle, instead of beneath 

 it as in Ichthyosaurus. The clavicle and interclavicle are the 

 only bones which have any close similarity of form in the two 

 types. The combined coracoid and precoracoid of the mono- 

 treme would not give the form of the coracoid bone in Ichthyo- 

 saurus, from which [there would be a notable difference in the 

 great thickness of the acetabular part of the bone. 



The pelvic girdle is less like that of a mammal. There is a 

 similarity in the ischium being larger than the pubis, in the narrow 

 pubis having a straight anterior border, and in its being (some- 

 times) anchylosed to the ischium to enclose an obturator foramen. 

 I do not remember any evidence whether the narrow cdrved iliac 

 bones were inclined forward or backward : they had no osseous 

 union with a sacrum. As a whole, the pelvis is probably least un- 

 like that of the monotreme, omitting from consideration the pre- 

 pubic bones, to which Ichthyosaurus has nothing corresponding. 



The humerus has a general resemblance to that of Cetaceans in 

 the shortness, strength, and compression of the bone, in the distal 

 end being formed of the flat inclined articular facets, in the proxi- 

 mal end being hemispherical, and in the flattened underside of the 

 bone being obliquely concave. The differences are, that in Cetacea 

 the outer trochanteroid ridges are suppressed, while those on 

 the inner side are so much developed, after the plan of the 



