\rl PISCES. 



side in the median line in front without the intervention of a premaxilla ; 

 and another small membrane-bone abutting on its postero-superior 

 margin, may be regarded as jugal (j.). The mandible exhibits three 

 bones in each ramus, the dentary (d) being much the largest, the articular 

 and angular (ag.} insignificant. No indications even of minute teeth have 

 been observed. The operculum (op.} is relatively small, the suboperculum 

 (s.op.) very large ; the branchiostegal rays (&;.) are about ten in number, 

 but there is no median gular plate. The notochord is shown (fig. 68) to 

 have been persistent, with no calcifications in its sheath, and the arches 



FIG. 67. 



Choiulrosteus acipenseroides ; restoration of head and pectoral arch, lateral 

 aspect, by E. H. Traquair, about one-quarter nat. size. L. Jurassic 

 (L. Lias) ; Dorsetshire and Leicestershire, ag., angular ; br., branchiostegal 

 rays; c.h., ceratohyal ; cl. , clavicle; d, dentary; /, frontal; h.m., hyo- 

 mandibular ; j, jugal; mx., maxilla; op., operculum; p, parietal; p.f., 

 postfrontal ; p.t., post-temporal; s.cl., supraclavicle ; s.o., suborbital ; 

 .i.op., suboperculum; s.t., supratemporal ; sq., squamosal. 

 o 



are only imperfectly calcined. There are no ribs. The pectoral arch is 

 almost identical with that of Adpenser, but the infraclavicle is relatively 

 smaller. The basal cartilages of the fins also resemble those of modern 

 Acipenseroids ; but there are no fulcral scales except on the upper caudal 

 lobe, where they form a single conspicuous series. The dorsal fin is 

 opposed to the pelvic pair, which are very remote. C. acipenseroides, the 

 typical species, occurs in the Lower Lias of Dorsetshire and Leicestershire, 

 and attains a length usually not exceeding one metre. A fish about twice 

 as large, associated with this, is named C. pachyurus ; but the genus is 

 not known from any other horizon. 



Gyrosteus mirabilis, from the Upper Lias of Whitby, must 

 have closely resembled Chondrosteus, but is known only by 



