238 
were only adapted to perform the same office in the economy of 
Nature in Mesozoic waters as their successors in the seas of the 
present day. The fundamental duties of life always remain the 
same ; fashions alone change. 
There is no more striking illustration of this fact, than that 
presented by the family of Pachycormide, to which most of the 
Ilminster fishes belong. To all outward appearance they are 
‘‘sword-fishes.” The bones of the snout consolidate and form a 
prominent rostrum, comparatively short in the Liassic Pachy- 
cormus, a little longer in some species of the Upper Jurassic 
Hypsocormus, longest and forming a veritable “sword” in the 
Cretaceous Protosphyrena. To give power to this rostrum, the 
body becomes rigid and spindle-shaped, and the tail forks into two 
long slender lobes. The pectoral fins become elongated, narrow, 
and stiff, to assist exclusively in balancing ; the pelvic fins almost 
or completely disappear. Now, the modern ‘‘sword-fishes” 
(Xiphiidz) possess a completely bony skeieton and the mechani- 
cal difficulties are readily overcome by the growth in size of the 
vertebrae, by their more firm union, and by the fusion of some of 
them to form a rigid fulcrum at the base of the powerful tail-fin. 
The Mesozoic Pachycormus and its allies, however, had a feebly 
ossified skeleton ; they were destitute of vertebrz, having only a 
persistent notochord with the usual slender arches above and 
below. The basis for the ‘‘sword-fish” type of trunk was thus 
entirely different, and the modifications for the required mechani- 
cal purpose are very interesting. The difficulty of the absence of 
vertebrz is overcome by the extreme multiplication of the arches 
above and below the notochord. The failure of all fusion of parts 
at the base of the tail-fin, is compensated for by the wide fan- 
shaped expansion of just one of the arches below the end of the 
notochord. 
These features are shown in the accompanying restored sketch 
of the Upper Jurassic Hypsocormus (Fig 2); and they can be 
especially well observed in the fine series of specimens of 
Tk 4. 
