LEPIDOTUS. 39 



consideral)ly into tlie lateral wall of the brain-case, and bears an articular surface 

 antero-superiorly (x.) probably for a small opisthotic, which is missing in the fossil. 

 Posteriorly it is fused with the pleurocentrum (pL) of the first ring- vertebra. 

 Inferiorly the exoccipital is firmly united with the large basioccipital {bo.), which 

 is grooved below for the basicranial canal, pierced behind by a deep excavation for 

 the notochord (n.), and fused with the large hypocentrum {Jnj.) of the first vertebra. 

 This hypocentrum bears a pair of prominent facets (r.) for the ribs. The pro-otic 

 bone of the same specimen is relatively large and must have articulated both with 

 the exoccipital and the opisthotic, while it was capped in front with a sphenotic 

 or postfrontal. 



It may be added that the epiotics unite in a median suture above the ex- 

 occipitals in some existing Teleostean fishes, such as Acdnflnirns. 



^ 



fr 



ec/tt. enji 



f 



Fig. 16. — Lepidotus mantelli, Ag-assiz ; transverse section of skull between orbits, two-thirds nat. size. — 

 Wealden: Hastings. Beckles Collection (B. M. no. P. 6342). ecpi , ectopteryg-oid ; e/yrf., ento- 

 pterygoid ; /"/•., frontal ; ol., interorbital passage for olfactory nerves ; pas., parasphenoid ; s., 

 interorbital septum. 



There are also ossifications in the interorbital septum, but their precise char- 

 acter is uncertain. A transverse section of one skull at the middle of the orbit is 

 shown in Text-fig. 16. In this position the septum ends above and below in very 

 coarsely cancellated bone, triangular in transverse section {s.), while the intervening- 

 part is merely a thin, ossified lamina, which widens in the middle into a tube, 

 doubtless for the passage of the olfactory nerves (o/.). Below the interorbital 

 septum the parasphenoid is seen (pas.), Avith the superficially ossified hinder part 

 of the entopterygoids (ewpt.) and ectopterygoids [ecpt.). Further forwards there 

 are indications of bone in the ethmoid region in several specimens, with apparently 

 two separate tubes for the passage of the olfactory nerves. 



The external bones are thick and vary much in appearance according to their 

 state of abrasion in the fossils. They are all more or less rugose, and unworn 

 specimens exhibit a variable extent of tuberculation. The parietal bones (PI. VIII, 

 fig. 2a; PL IX, figs. 1, 1 a; PI. X, fig. 3; pa.) form an unsymmetrical pair, of 

 which one is both wider and longer than the other; they unite in a nearly straight 



