THE MORMTRID^, NOTOPTERIDiE, AND HYODONTID^. 193 



is no spicular bone. The third pharyngobranchial is not longer 

 than the second, and overlaps it only to a very slight extent. 

 The four epibranchials are approximately equal in length, and 

 the fourth is not expanded. 



Peteocephalus bane. 



Cranium (PI. 23. figs. 6, 7, and 8). — Probably the most remark- 

 able feature about the cranium of PetrocejpTialus is the unusual 

 height of the mesethmoid region. The mesethmoid lies below 

 the frontal bones, the anterior wings of which extend to the 

 extreme front of the cranium, and its lower part is set well in 

 advance of the vomer. The latter bone is small and edentulous, 

 and is fused at its sides with the mesial faces of the two 

 palatine bones. The parasphenoid is of considerable vertical 

 height in its anterior part ; it carries on its ventral surface a 

 lanceolate patch of some thirty teeth, which are single-pointed, 

 and thus differ from those of the jaws. The parasphenoid 

 extends as far backward as the middle of the under surface of 

 the basioccipital ; there is no posterior opening to the eye-muscle 

 canal. 



The articular groove for the reception of the hyomandibular 

 is set low down the side of the cranium, and the pro-otic is four- 

 sided and faces downward, instead of downward and outward. 

 Between the epiotic ridge and the squamosal ridge is a large 

 aperture leading into the cavum cranii in the dried skull, but in 

 nature lodging a nearly spherical thick- walled vesicle. The 

 aperture is bounded by the exoccipital, epiotic and squamosal. 

 There is no opisthotic. The supraoccipital has a strongly 

 developed median crest ; the two parietals meet in the middle 

 line, and at their junction form a low crest, continuous with that 

 of the supraoccipital. 



The frontal bones are rather small, and each has a slender bar 

 of bone arching obliquely over it. The prefrontals are small 

 but distinct ; they do not touch one another in the median 

 plane. A basisphenoid is present, and is united above with the 

 orbitosphenoid and alisphenoid, and below with the para- 

 sphenoid. The orbitosphenoid is larger than the alisphenoid, and 

 extends from the frontal above to the parasphenoid below. 



Temporal and Preopercular Series (figs. 5 and 9). — The post- 

 temporal resembles that of Mormyrops. The supratemporal 



LIIW. JOUEN. — ZOOLOaX, VOL. XXIX. 14 



