A PYCNODONT FISH OF THE GENUS MESODON. 185 



series in the scaly part of the trunk, but they do not appear to 

 reach the humped dorsal margin. Traces of the ribs show that 

 they were also long and stout. The middle part of the vertebral 

 column is destroyed, but the terminal half of the caudal region 

 is well preserved. In the four segments in advance of the tail 

 fin the neural are about as large as the haemal arches ; but 

 within the fin there is the usual expansion and preponderance of 

 the haemals. The condition of the vertebral column suggests 

 that there has been no distortion of the abdominal region, but 

 probably a little in the anterior half of the caudal region. 



The pectoral fins are not shown, but remains of the very small 

 pelvic pair (plv.} are preserved remarkably far forwards. There 

 are indications that the pelvic fin-supports are a pair of long and 

 narrow laminae, while the fin-rays must have been few, broad, 

 and closely articulated. The rays of the median fins are also 

 broad and closely articulated to a point not far from the base. 

 The dorsal fin seems to have 30 supports, but its shape is not 

 seen. The anal fin, with "19 or 20 supports, rapidly rises to a 

 point anteriorly, its maximum depth equalling about three- 

 quarters of its basal extent. The caudal fin is obscure, displaying 

 only the short crowded rays at its origin below. 



The scales are confined to the trunk in advance of the median 

 fins, and, so far as preserved, they are in regular undisturbed 

 series. Some are seen from within, others in impression of the 

 outer face. Their inner face (PI. B, Fig. 3), bears a low and 

 broad vertical ridge, ending in a large peg-and-socket articu- 

 lation. Their outer face (PI. B, Fig. 4), is sparsely ornamented 

 with large rounded tubercles, which are hollow. The scales of 

 the middle portion of the flank are not less than twice as deep as 

 broad, while those near the dorsal and ventral borders are more 

 nearly equilateral. The dorsal and ventral ridge-scales are 

 ornamented with stout enamelled prickles. 



The Portlandian Pycnodont now described is clearly referable 

 to the genus Mesodon. The characters of the splenial dentition, 

 the squamation, and the short tail-pedicle leave no doubt as to 

 this determination. The only difficulty is in deciding upon the 



