THRISSOPS. 137 



p. 12; vol. ii, pt. ii, 1844, p. 124, pi. Ixva), from the Lithographic Stone (Lower 

 Kimmeridgian) of Bavaria. 



Remarks. — Thrissops seems to range in Europe from the Middle Jurassic 

 (Oxfordian) to the Lower Cretaceous ; but without a knowledge of their cranial 

 osteology, the systematic position of the later species usually ascribed to this genus 

 must remain uncertain. They closely approach the Ichthyodectidae or Chirocen- 

 tridae. 



1. Thrissops curtus, sp. nov. Plate XXVI, fig. 1. 



Type. — Imperfect fish ; British Museum 



Specific Character*. — At least 1G cm. in length. Length of head with opercular 

 apparatus nearly equalling maximum depth of trunk, and contained from four to five 

 times in total length of fish ; maximum depth of trunk contained about two-and-a- 

 half times in length from pectoral arch to base of caudal fin. Pelvic fins half as 

 large as pectorals, inserted nearer to the anal fin than to the latter ; dorsal fin, with 

 13 rays, arising behind the origin of the anal fin, which is slightly more elevated in 

 front and comprises 30 rays ; caudal fin very deeply forked and lobes slender. 



Description of Specimen. — The type and only known specimen (PI. XXVI, fig. 1) 

 lacks the jaws, but is otherwise nearly complete. In the skull the ethmoidal region 

 is very short and small, and the parasphenoid is seen crossing the relatively large 

 orbit. A short ridge inclined upwards at the back of the occiput may perhaps be 

 the thickened upper border of a triangular supraoccipital crest such as seems to 

 occur in Thrissops formosus (Brit. Mus. no. P. 917). The preoperculum is vaguely 

 shown to be much expanded at the angle and marked with several fine radiating 

 ridges. The operculum, which is smooth, is widest below, and its maximum width 

 is at least three-quarters of its depth. It seems to show a peculiar slime-pit near 

 its point of suspension like that already described in Pachytlirissops Isevis (p. 131, 

 PI. XXV, fig. la). The suboperculum, which is also smooth, is slightly more than 

 one-third as deep as the operculum. The vertebral centra are evidently well 

 ossified, but are not clearly seen except at the end of the tail, where they bear 

 the usual sharp median lateral ridge. Over twenty vertebrae can be counted in the 

 caudal region. The neural arches in the abdominal region are separated from the 

 neural spines, which reach the dorsal border. Some of the anterior spines of this 

 series have a small laminar expansion at their lower end. The stout grooved 

 ribs, of which there are over twenty pairs, reach the ventral border. The neural 

 and haemal arches in the caudal region are fused both with their spines and with 

 the corresponding centra; they are slender, short, and much inclined backwards. 

 At least nine haemals are included in the base of the caudal fin. Intermuscular 

 bones occur only in the dorsal part of the abdominal region. Behind the skull, 

 the post-temporal is a large, smooth, rhomboid plate, nearly as wide as deep, 



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