Chap. II. THE CIIELYDROID^. 343 



The marginal rim consists of eleven pairs, besides the odd ones at the ends, just 

 mentioned. In the plastron there are nine plates, four pairs and one odd one. 

 The second and third pairs unite with the marginal rim, form the narrow bridge, 

 and then, stretching out lengthwise, form the larger part of the whole plastron. 

 The first pair meet at the front end before the attachment of the shoulder 

 apparatus, under the neck, where they are broadest, and then growing narrow, 

 reach backwai-d and outward and overlap the outside of the second pair. The odd 

 plate is quite small ; it is situated just back of the first pair within their angle, 

 and sends a slender slip back some distance between the inner edges of the 

 second pair. The fourth jiair meet under the pelvis, terminating in a point just 

 behind it, and reach forward and outward and overlap the third pair ; they are 

 broad where they meet, and grow narrow forward. 



The scapular arch is high, and nearly perpendicular ; it is much higher than 

 broad, so that the shoulders are not nearly as wide apart as in the Trionychidoe, and 

 not so near the outer edge ; the coracoid process, the acromion, and the scapula 

 are all long, especially the latter ; the coracoid pi-ocess is broad at its ends. The 

 sacrum is Inroad ; the iliac bones reach far forward, and approach each other as 

 they descend from the sacrum, so that the hip joints are placed under the body far 

 inward of the outer edge of both the end and the sides of the shield ; the pubis 

 and ischium reach steeply downward, and the processes of the pubis, which are 

 long and strong, reach downward and forward, and not sidewise. The legs and feet 

 are large and strong, the toes are stout, and all but the outer one of the hind 

 feet terminate in long, curved, sharp, strong claws ; they are freely flexible, liut 

 not capable of being spread nearly as wide apart as those of the Trionychidte, 

 and the web is much smaller, the w'hole foot being more compact than in the 

 latter family. 



The dors;d vertebral column is deep from the shield downward, and there is a 

 large space for the longissimus dorsi on cither side of it above the ribs for its 

 whole length; the size of this space is connected with the flattening of the shield 

 above. The isolated true bone, situated at the front end of the body, is quite 

 distinct and prominent ; it sends long, slender arms on either side under the mar- 

 ginal rim, as far l)ack as to the ends of the second pair of ribs. 



The neck is long, flexible, and stout, and has a i)owerf'ul luuscular apparatus. 

 The tail, or, more properly speaking, that part of the vertel)ral column which extends 

 behind the s.acrum,^ is very long and strong, much longer than the column between 

 it and the neck. This is the case in the American genera, at least. 



' The great length and strcnglli of tliat part of ernm is not simply to he considered as relating to 

 the vertebral eolunin which extends beyond the sa- the size of the tail : the part which this region 



