382 
ACTIN' OPTERYGII . 
are perhaps rare traces of slight ossifications in its sheath both in 
the abdominal and caudal regions. The neural and haemal arches, 
however, are all well ossified, numerous, slender, and very closely 
arranged ; while the lamina of each arch is remarkably deep and 
narrow, and might readily be mistaken for a vertebral element. The 
ribs are especially slender, not extending to the ventral border of the 
abdomen ; and the sigmoidally-bent neural spines in the abdominal 
region remain separate from their supporting arches, fusion not 
taking place until a point considerably behind the origin of the 
dorsal fin. In the caudal region all the spines are firmly fused 
both with the neural and haemal arches, while in its hinder half 
they increase in robustness and in backward inclination. It is also 
noteworthy that one haemal arch develops into a great fan-shaped 
expansion, for the more adequate support of the powerful caudal 
fin. There are no intermuscular bones. 
The clavicle is large and extends far forwards in the gular region 
to meet its fellow of the opposite side. There are also large post- 
clavicular plates. The rays of the pectoral fin are very closely 
apposed, and only divided quite at the distal extremity ; its anterior 
margin sometimes exhibits a fringe of relatively minute fulcra. No 
pelvic fins have been observed in any species. The rays of the 
median fins are very slender and exhibit distant articulations, 
which sometimes appear as if incomplete or secondarily fused. At 
the anterior margin of each fin the rays gradually increase in length 
to its apex; but there are very rarely traces of minute fulcra 
between their successive points. A few free fin-supports occur 
above the neural spines in advance of the dorsal fin. 
The scales are all thin and very deeply overlapping, nob strength¬ 
ened by ribbings within or united by peg-and-socket. On the flank 
the rhombic exposed areas are about as deep as broad, while towards 
the ventral border a few are broader than deep. There is one 
greatly enlarged scale near the origin of the anal fin. The lateral 
line is distinctly traceable on the flank, and in a few specimens it is 
shown to bifurcate irregularly on some of the scales in the abdo¬ 
minal region. 
The form of the trunk in Pachycormus is such that it assumes 
very varied outlines when crushed. Moreover, the apparent size of 
the scales differs greatly according to the degree and manner in 
which they are displaced. The determination of the species is thus 
very difficult and uncertain, and many more forms have been named 
than it is possible to define. In the following provisional arrange¬ 
ment the relative depth of the trunk is thus ignored in the diagnoses, 
