104 REPORT — 1839. 



the sterno-costal arc would be reduced to the same number of 

 elements as in the Crocodile : if the lateral styles were taken 

 away, it would i-esemble the simple sterno-costal arc of the 

 Polychriis. 



The Pectoral Extremity. — We have already remarked that 

 the extremities of the JEnaliosawi offer the nearest resemblance 

 in their bony structure to the paddles of the Cetacea. But this 

 resemblance is limited to the radiated system of bones, i. e. to 

 the hrachium, antibrachium and mcmus. The mode in which 

 the locomotive member is connected with the trunk is entirely 

 different in the two aquatic tribes. In the Cetacea the pectoral 

 fin is attached to a simple scapula with a rudimental acromial 

 and coracoid process, and is merely suspended in the flesh. In 

 the Ichthyosaurus y as in the Plesiosaurus, the pectoral fin is 

 connected with, and must have acted upon a powerful and re- 

 sisting osseous arch, having the sternum for its keystone. The 

 sternum in fact here exists solely for the function of the anterior 

 members, and does not enter at all into the formation of the 

 costal arches or the respiratory cavity. In the Cetacea on the 

 contrary the sternum is limited to its connection with the ribs, 

 and to the completion of the thoracic receptacle of the large 

 and highly developed lungs. 



In the Ichthyosaurus the representative of the sternum is 

 analogous to the episternal element as it exists in the Orni- 

 thorhynchus and Lacertian /S'«?«n«, and, as in many of the latter 

 tribe, it presents a triradiate form. One branch occupies the 

 median line of the pectoral arch, is broad and flat, and rounded 

 posteriorly; the other two rays branch off from each of the 

 anterior angles of the median piece, and, diverging laterally, fol- 

 low the curvature of the superimposed clavicles, to the posterior 

 and middle part of which they are closely atached : as they pro- 

 ceed outwards, these lateral rays of the episternal bone gradually 

 diminish to a point. 



The scapula is a short but stout and broad bone presenting 

 the simple parallelogramical form which characterizes it in the 

 Oviparous Vertehrata. The anterior margin is fixed to the 

 clavicle and to the extremity of the lateral process of the epister- 

 num : the inferior extremity presents two facets, one of which is 

 attached to the coracoid bone, the other forms part of the arti- 

 cular surface for the humerus. 



The coracoid bones, which constitute at their contracted and 

 thickened outer extremities the remainder of the glenoid ca- 

 vity, become suddenly and remarkably thinner and expanded 

 as they pass inwards to articulate with the episternal bone. 

 They are also complicated each in the young Ichthyosauri with 

 an epiphysial piece wedged into the angle between their ante- 



