116 PISCES. 



strengthened by secondary peripheral calcifications in the later species. 

 The ribs are delicate and do not reach the ventral border of the fish ; 

 the neural arches in the abdominal region are not firmly fixed either 

 with the centra or with their appended spines ; but both neural and 

 hsemal arches and spines are fused with the centra in the caudal region. 

 The haemal arches at the base of the tail are thickened, but not much 

 expanded and never fused together. The short dorsal fin is opposed to 

 the pelvic pair or immediately behind ; the caudal fin is forked. The 

 typical and earliest undoubted species of Leptolepis is L. bronni from the 

 Upper Lias of Wiirtemberg, and nearly similar or identical fishes occur 

 on a corresponding horizon in N. France and Somersetshire, Gloucester- 

 shire, and Yorkshire. L. macrophthalmus, from the Oxford Clay of 

 Christian Malford, Wiltshire, is a well-known species about 0'3 m. in 

 length. Several species occur in the Lithographic Stone (Lower Kim- 

 meridgian) of Bavaria, and one diminutive form, L. brodiei, is known 

 from the Purbeck Beds of the Vale of Wardour, Wiltshire. The genus 

 is also represented in the Upper Hawkesbury-Wianamatta Beds (Lower 

 Jurassic) of Talbralgar, New South Wales. 



Thrissops. Differing from Leptolepis in its more upturned mouth, 

 want of thickening in the elevated part of the dentary, larger and 

 stouter neural spines and ribs in the abdominal region, more extended 

 anal fin, and the dorsal opposed to the latter. This is an Upper 

 Jurassic and Lower Cretaceous genus, the typical species being T. for- 

 mosus, 0'5 m. in length, from the Lithographic Stone of Bavaria. Other 

 species occur on the same horizon in Bavaria, Wiirtemberg, and Ain, 

 France, and fragmentary remains of a very large species are also met 

 with in the Kimmeridge Clay of Dorsetshire. T. portlandicus is found 

 in the Portland Stone of the Isle of Portland, and other species are 

 represented in the Purbeck Beds of Dorsetshire. T. microdon occurs in 

 the Lower Cretaceous of Comen, Istria, and the Isle of Lesina, Dalmatia, 

 where other species have also been determined. 



The Oligopleuridae are exclusively Upper Jurassic and 

 Cretaceous fishes of moderate size. In outward aspect the 

 head much resembles that of the Pholidophoridae and Lepto- 

 lepidse, the arched maxilla being bounded above by the two 

 'supramaxillaries ; while the dentary bone of Oligopleurus itself 

 is known to be almost identical with that of Leptolepis. The 

 vertebral centra are always completely ossified ; the ribs are 

 small ; and there are never intermuscular bones. The haemal 

 arches at the base of the tail are only thickened, never much 

 expanded and none fused together. The fins are fringed with 

 fulcra. The scales are cycloidal and deeply overlapping. 



