INTRODUCTION. 
XI 
Here, again, the earliest term in the series {Opliiopsis) is the 
most generalized; and it has the most extensive range (Upper 
Trias—Purheckian). It is, indeed, a distinct link between the 
family of Macrosemiidae (to which this Catalogue assigns it) and 
that of the Eugnathidae to be considered below. The Lower Kim- 
meridgian and Purbeckian species are also interesting as exhibiting 
ring-vertebrae, the separate, alternating pleurocentral and hypo- 
central rings being sometimes observable in part of the caudal 
region. 
The diminutive Ehaetic Legnonotus , with greatly extended dorsal 
fin, is a more pronounced member of the Macrosemiidae. This has 
an apparently regular squamation; but the closely-related genus 
Macrosemius (Lower Kimmeridgian, Portlandian, and Purbeckian) 
is remarkable for the degeneration of its scales, which are very thin 
and subdivide towards the dorsal and ventral margins, being usually 
wanting near the former. The dwarfed species of Portlandian and 
Purbeckian age may perhaps have been entirely scaleless. There are 
no fulcra except on the caudal fin; but the rays of the dorsal fin 
in some species are curiously denticulated on the hinder border. 
Traces of vertebral centra are never observable. 
Histionotus is a thick-scaled contemporary of Macrosemius , with 
fulcra on all the fins and with delicate ring-vertebrae. Propterus 
and Notagogus , of the same age, are characterized by the subdivision 
of the dorsal fin into two parts; and the former has thinner scales 
and more delicate vertebrae than the latter. 
The late Cretaceous genus Petalopteryoc is an excessively elon¬ 
gated fish with irregularly arranged rhombic scales, and the robust 
cheek-plates subdivided into small tesserae. 
The two most characteristic features of the Macrosemiidae—the 
elongated trunk and excessively extended dorsal fin—are thus 
developed early in a Phaetic fish with normal squamation ( Legno- 
nolus) ; ring-vertebrae are only well-formed in the genera which 
retain comparatively thick scales ; the scales and fulcra begin to 
degenerate in the Upper Jurassic {Macrosemius); the scales, though 
still rhombic, are irregularly subdivided in the Cretaceous Pctalo- 
pterjoc , and in this unique genus the cheek-plates are represented 
by an investment of small tesserae. 
, Pycnodontidce. 
A curious parallelism will be noticed in the development of the 
Semionotidae and Macrosemiidae as thus briefly sketched. Yertebral 
