58 



ones disappear in the twelfth : the metapophyses have subsided in 

 the fifteenth. The neural canal is unclosed above in the sixteenth ; 

 and the vertebra is reduced to its central element in the last two 

 caudals. 



The skull is remarkable for its small proportional size, for its long 

 and slender cranial portion, its large and complex zygomatic arches, 

 its broad truncate facial part, with the slender produced premax- 

 illaries, and for the great depth of the middle of the lower jaw. 



The mastoid element developes a large tuberous process and a 

 deep semicircular articular cavity for the stylohyal. The malar 

 bone sends down a long process outside the lower jaw. The num- 

 ber of teeth is ^3^=18, the fifth in the upper jaw being the smallest. 

 They are alike in structure, and differ but little in shape: the grinding 

 surface in most is crossed by two transverse ridges, the summits of 

 which are formed by hard dentine ; the rest of the tooth being 

 composed of a central body of vaso-dentine and a peripheral mass 

 of vascular cement. The microscopic characters of these several 

 constituents of the teeth were then described. Each tooth is deeply 

 implanted in the jaw, where it terminates without dividing into 

 fangs, by a widely open pulp~Cc\vity for a persistent matrix, ensuring 

 perpetual growth. The stylohyal bone has the form of a hammer, 

 with a long, slightly bent handle ; one part of the head being thick- 

 ened and rounded for articulation with the cavity in the mastoid. 



The scapula presents almost the form of a trapezium, with the 

 inferior angle bent outwards, increasing the depth of the subspinal 

 fossa : there is a rudiment of a second spifle, below the normal one : 

 the acromion is expanded, produced and confluent with the coracoid ; 

 and the supraspinal fossa is perforated by a circular aperture. The 

 clavicle has a well-marked sigmoid flexure, equally-developed obtuse 

 extremities, without any articular surface. The humerus is remark- 

 able for the enormous development of ridges for the attachment of 

 the muscles, especially at its distal end : the inner condyle is not 

 perforated as in the Megalonyx ; it is devoid of a medullary cavity. 



The ulna and radius are next described. The carpus consists of 

 seven bones, three of which are proper to the first row, three to the 

 second, and one is common to both; the latter answers to the ^sca- 

 phoides' and ' trapezium' in the human wrist, and articulates with 

 the radius above, and the rudiment of the metacarpal of the pollex 

 below. 



Only four digits are developed, the first or * pollex ' being obso- 

 lete. The 'index' or second digit has three phalanges, the last sup- 

 porting a large claw, and being twice as long as the two preceding 

 phalanges. The proximal and middle phalanges of the ' digitus 

 medius' are confluent. The ungual phalanx is shorter than that of 

 the index, but has twice its vertical breadth. The metacarpals 

 progressively increase in length from the first to the fifth. The 

 fourth digit or 'annularis' has three phalanges, the last being un- 

 guiculate and longer than that of the ' medius.' The fifth digit 

 has only two very short rounded phalanges, which v/ere doubtless 

 buried in a thick callous outer border of the foot, on which the 

 Megatherium rested when applying the foot to the ground. 



