TRACHICHTHYOIDES. 29 



Genus TRACHICKTHYOIDES, iiovuiii. 



Generic Characters. — Frontal region of skull depressed and flattened, with a 

 shallow, antero-posteriorly elongated, rhombic depression occupying its middle 

 portion ; ethmoidal region very small ; mouth small, with minute clustered teeth ; 

 orbit of moderate size ; preoperculum without spine, only serrated ; operculum 

 irregularly triangular in shape, its hinder margin being produced into one very 

 prominent broad spine. Scales pectinated, extending over the cheek. 



Type Species. — Tracliiclithyoides ornatus, from the Chalk of Kent. 



Bemarhs. — This genus is as yet known only by the head, which seems to have 

 been very similar to that of the existing genera TraclticJttln/s and Gepliijroherij.f, l)ut 

 lacks the preopercular spine. 



1. Trachichthyoides ornatus, sp. nov. Plate VIII, fig. 5. 



Tijpe. — Imperfect head ; British Museum. 



Specific Characters. — Maximum width of cranium slightly exceeding half its 

 length ; cranial ridges and supraorbital border finely rugose and tubercular. 

 Maximum width of operculum equalling two-thirds of its depth. 



Description of Specinien. — The unique fossil on which this species is founded, 

 is shown of the natural size from the left lateral and suj^erior aspects in PL VIII, 

 figs. 5, 5 a. The cranium is well preserved, shown a little obliquely in fig. 5, 

 directly from above in fig. 5(/. The parietal-squamosal region is very short, and 

 there is no supraoccipital crest rising above the plane of the flattened cranial i-oof. 

 The occipital plane slopes backwards and downwards. The large frontal bones 

 (/r.) are divided by a persistent median suture, and do not taper in front, where 

 each is obliquely truncated. Each expands at the outer side into a supraorbital 

 flange, and this is connected by a ridge Avith a bent longitudinal ridge, which 

 bounds a shallow slime-cavity in the middle of the cranial roof. Another short 

 ridge extends from the chief frontal ridge to the pterotic region ; and on the floor 

 of a shallow slime-cavity in the anterior half of the frontal bone an oval vacuity is 

 conspicuous. The supraorbital Ijorder and all the ridges are ornamented with 

 fine though prominent tubercles and vermiculating rugie. A fragment of the 

 mesethmoid [etli.) proves this element to have lieen comparatively small and 

 narrow. Remains of the circumorbital ring of cheek-plates {co.) exhibit a broad, 

 everted orbital rim, which is ornamented like the ridges of the cranium and over- 

 hangs a deep channel for the slime of the sensory apparatus. The largest plate 

 beneath the eye is serrated on its loAver edge. Behind and beloAV the circumorbital 

 ring, the cheek is covered with strongly pectinated scales (.?.). The depth of tlie 



