238 PEOCEEDINGS OF THE ACADEMY OF 



the tibia is excessively oliliqiie, requiring that one or both sections of the limb 

 should be very oblique to the vertical line. As the tarso-metatarsal elements 

 fcupport the weight immediately on the ground, and as it is obvious that the 

 leverage moving the great weight of the body on its support must have been 

 tiie gastrocnemius and soleus muscles extending the tibia on the metatarsal 

 segment as the fixed point, and as there is no indication of correspondingly 

 powerful muscles to flex the metatarsals on the phalanges, it is obvious that 

 the latter has been the more vertical, and the former the more oblique seg- 

 ment. And if the tibial segment has been obliciue, for reasons just given, the 

 femur miirn have been oblique also.* 



The length of the femur has had relation to another peculiarity as well, as 

 follows : 



In an animal designed to walk erect, it is necessary that the centre of gravity 

 should be transferred as far posteriorly as is consistent with the type. In Lte- 

 laps and other Dinosauria w^^e have very elongate pubic and iliac bones, and, as 

 I have before described, these appear to have been designed to enclose and 

 support an abdominal mass, in a position beneath the sacrum, and posterior to 

 the position observed in quadrupedal mammals and reptiles. We would thus 

 have a prominent keeled belly between the femora, supported by elongate 

 curved ischia behind, and slender pubes directed downwards in front. In Poe- 

 cilopleurum the space between the latter and the sternum was occupied by 

 abdominal ribs. The length of femur places the arc through which the knee 

 moves beyond this projection. 



The confluence of a greater number of vertebra; to form a sacrum, seen in 

 this order and in the birds, would seem to have a direct relation to the support 

 of the above-mentioned greater weight by it, than in horizontal vertebrata, 

 where the weight is distributed throughout the length of the vertebral column. 



The shifting of the neural arches backwards, seen in the same orders, pointed 

 out by Owen, would have a mechanical relation to the same necessity, — i. e., 

 their partial transfer over the intervertebral spaces naturally tending to 

 strengthen the union of the sacral elements. 



The foot need not, however, have been placed precisely beneath the centre 

 of gravity of the body, as the animal was furnished with a tail of greater or less 

 weight. This member bears, however, little proportion to the great size of 

 those seen in Iguanodon, Hadrosaurus, etc., but exhibits a commencement of 

 the reduction which is so striking among the birds. 



The proportions of the metatarsus are only to be ascertained by an examina- 

 tion of those of allied species, as L. m ac ropu s and Megalosaurus b u c k- 

 I an d ii. As all the other bones are more slender than those of the latter, so 

 were no doubt these bones longer in proportion to their breadth. I have esti- 

 mated it above as equal to a little over half the tibia. 



The digits in the genus Ladaps have not, in all probability, been more than 

 four. The less bird-like forms of HyUeosaurus and Iguanodon have had, ac- 

 cording to Owen, but three metatarsals, and it is^ot according to the rule of 

 successional relation that there should be any repetition of a reptilian character, 

 in a iioint of prime importance in measuring the steps of succession between 

 re]jtiles and birds. Lselaps, and probably Megalosaurus also, had but three 

 digits directed anteriorly, and a fourth lateral or rudimental. 



It is true that Deslongchamps ascribes five digits to Poecilopleurum, after a 

 cafeful study of abundant material. He was, however, much more impressed 

 ■with the Crocodilian affinities of that reptile than with any other, a,nd did not 

 recognize the avine in the astragalus. It seems to me quite possible that one 

 of his toes can be 'dispensed with, — for example, the second, of which but one 

 phalange is said to remain. If we ascribe the fractured extremity of the bone 



* Probably in a squ.itting posture the animal rested on the entire sole as far as the heel, 

 though not under ordinary cireumstauces ; as I have suggested in Amer. Katuralist, 1, 28, 

 Mycteria and other wading birds assume a similar position at times. 



[Oct. 



