412 NATURAL SCIENCE. June, 



older geneva., Liodesmns and Megaliirus, begin in the Jurassic ; the third, 

 Amia, begins in the Lower Chalk and survives to the present day. 



A. S. Woodward regards the Pachycormidae, which appear first 

 in the Upper Lias, as a remarkable modification of the thin-scaled 

 Eugnathidae. The vertebral column of these fishes, which are mostly 

 large, is divided into very numerous short segments, but incompletely 

 ossified, being formed either of half- vertebrae or ring- vertebrae. It 

 turns upwards at the hinder end, without, however, being continued 

 into the upper lobe of the powerful, deeply-cleft tail-fin ; an unusually 

 expanded haemal arch at the basis of the lower lobe shows, notwith- 

 standing the complete outward homocercy, the typical structure of 

 the Protospondylic tail. The strong pectoral fins indicate a remark- 

 able power of swimming. In the older genera [Sanvopsis and Euthynotus) 

 the jaws above and below are provided with stout conical teeth and 

 are rounded off in front ; Hypsocormis and Pachycormns are characterised 

 by the strong development of the dentition, and in one species of 

 Hypsocormns from the Oxford Clay of Peterborough the premaxilla is 

 elongated into a prominent rostrum with powerful teeth, as in the 

 Cretaceous genus PTO/o5/'/y/y^;za, where it becomes a long conical process. 

 The union of this hitherto incompletely known genus with the 

 Pachycormidae is somewhat surprising and still needs further proof. 



In the extensive order of Isospondyli Cope places a large number 

 of true bony fishes from the Cretaceous, the Tertiary, and the present 

 day. Woodward partly follows the celebrated American palaeonto- 

 logist, and deals in the volume before us with the three oldest families, 

 the members of which Agassiz assigned to the Ganoidei. Until now 

 indeed, the Pholidophoridae have been generally recognised as 

 ganoids. Like the majority of the Protospondyli their body is 

 covered with rhombic ganoid scales articulated together ; the 

 vertebral column consists chiefly of ring-vertebrae ; on the other hand, 

 the want of a splenial and a coronoid element in the lower jaw points 

 to their inclusion in the Isospondyli. The few genera are distributed 

 through the Trias and Jurassic. The closely-related Oligopleuridae 

 range from the Jurassic to the Upper Chalk. In many respects they 

 are suggestive of the Amiidae ; their scales are thin and cycloid, their 

 vertebral column is completely ossified, but not with separate pleuro- 

 centra and hypocentra like those of Amia in the caudal region. The 

 Leptolepidae have already been separated from the group of ganoids 

 by Kner, Thiolliere, Liitken, and most authors, and assigned to the 

 Teleostei. They are very sharply separated from the Pholidophoridae 

 and Oligopleuridae by the want of fulcra on the fins and by a con- 

 siderable development of inter-muscular bones. The vertebral 

 column is also completely ossified, consisting of amphicoelous 

 vertebrae, and the tail is both internally and externally homocercal. 

 The Leptolepidae are remarkably similar to the Clupeidae, but are 

 distinguished from the latter by the parietal bones meeting in the 

 middle line, by the simple, separate haemal spines at the base of the 

 tail, and by the stronger development of the enamel on the scales. 

 The genera Leptolcpis, Aethalion, and Thvissops, indeed, form the 

 passage between the ganoids and the teleosteans. 



The sharp definition of the principal groups (sub-classes and 

 orders) within the great classes of the animal kingdom presents great 

 difficulties, not only among the fishes, but also among the Invertebrata ; 

 we are, indeed, well aware that in reference to the rational sub- 

 division of the Anthozoa, Crinoidea, Cystoidea, Brachiopoda, 

 Lamellibranchiata, Gastropoda, Cephalopoda, etc., there are as wide 

 differences of opinion among authors as in reference to the prime 



