ICHTHYODECTES. 95 



.sloping upper part of the otic region. The postfrontal or sphenotic ossification 

 (ptf.) is rather large, and is exposed in the somewhat hollowed anterior part of this 

 region. When the otic bones are viewed from the side beneath the overhanging 

 pterotic (as in PI. XIX, fig. 5 a), they are observed to be excavated in the middle 

 by a large and deep fossa (fo.). A moderately large opisthotic (opo.) occurs above 

 the basioccipital (bocc), and its anterior edge is suturally united with the pro-otic 

 {pro.). This element is very large and pierced by the usual foramen for the exit 

 of the fifth nerve. The pterotic, sphenotic, and pro-otic enter the hyomandibular 

 facette (hm.). The parasphenoid (Text-fig. 22 d, pas.) forms the floor of a very 

 large basicranial canal and extends upwards on either side in a short and broad 

 process, which adjoins the front part of the pterotic. The basicranial axis formed 

 by the parasphenoid is only slightly arched, without any sharp bend at the anterior 

 termination of the otic region. The prefrontal or ectethmoid (prf.) is remarkably 

 massive for articulation with the thickened palatine, which supports the maxilla. 



Among minor characters shown in Text- figs. 22 a — d, it may be noted that the 

 skull of Ichthyodectes differs from that of Chirocentrus in the relatively greater 

 extent of its epiotic element, the larger size of its hinder supraoccipital crest, and 

 the very slight elevation of its pterotic crest (u). 



Vertebrse. — None but small fragments of vertebral column have hitherto been 

 found in the English Chalk. All the centra, except a few at the two extremities of 

 the column, are pitted on each side by two deep, longitudinally- extended 

 excavations (PL XXI, fig. 3) ; while they also receive the bases of the neural and 

 hasmal arches in pits. Some of the terminal caudal vertebra3 have already been 

 figured by Agassiz 1 and Dixon 2 under the name of Tetrapterus minor, on the 

 erroneous assumption that they belong to the fish whose rostrum is now known as 

 Protosphyr&na minor. A more satisfactory specimen of five vertebras in natural 

 sequence is shown in PI. XXI, fig. 4. The centra (c.) of these terminal caudals are 

 closely pressed together, with rugose sides which exhibit only a small remnant of a 

 pit. As seen in end-view in the specimen figured by Agassiz, they are pierced by 

 a small foramen for the passage of a persistent remnant of the notochord. The 

 procumbent neural arches (n.) are relatively small and not much expanded at the 

 base. The hasmal arches (It.), mistaken by Agassiz for neurals, are remarkably 

 stout and large, with a basal expansion which firmly clasps but is not fused with the 

 centrum. The hasmal canal is diminutive and (as shown by Dixon's figure) the 

 arch encircling it is much wider than long. The sides of each hasmal spine are 

 flattened and marked with some faint oblique ridges. The haemal spines posterior 

 to those here described are small triangular expansions. 



Scales. — The scales proved by the Kansas fossils to belong to Ichthyodectes and 



1 L. Agassiz, Poiss. Foss., vol. v, 1837, pi. lxa, figs. 11 — 13. 



2 F. Dixon, Geol. Sussex, 1850, pi. xxxi, fig. 16. 



