193 



The spine supported by the scapular arch in Portheus and Ichthyodectes, 

 is a defensive weapon. Proximally, it presents a concave articular surface for 

 the scapula, with a short hook- like projection bounding one end. A specimen 

 in relation, but somewhat mutilated, exhibits a flat, discoidal basilar bone, 

 which is probably applied to one of the scapular facets. Two rod-like 

 basilars are visible, and two round condyles, projecting from the mass at the 

 base of the fin-rays, fit into the scapular cotyli. The principal spine is flat 

 and curved ; the convex edge trenchant beyond the middle. The posterior 

 edge is obtuse but narrow, and exhibits a slight groove on one side medially. 

 Proximally, there is a shallow rabbet, whose floor is transversely rugose. 

 Several layers of -the tissue of the spine beyond the basal portion are deli- 

 cately longitudinally striate. The distal half is broken away. Length of 

 fragment, 1 foot ; width, 1.5 inches ; thickness at middle, 5 lines. The 

 largest pectoral spine of Portheus in my possession measures 2.75 inches 

 in width. 1 



The vertebras in this genus are rather short, but not so much so as in 

 sharks. In P. thaumas, nearly eighty dorsals* and caudals were preserved ; 

 those without lateral grooves, or cervicals (the term not appropriate), are not 

 numerous. There are not more than three vertebrae entering the caudal 

 fin ; a fact which is difficult to determine, owing to the concealment of the 

 terminal centra by bases of radii. There are seven haemapophyses in the, 

 support, all flat except the first, which is like those anterior to it. The sec- 

 ond is articulated freely to its centrum," and is wider than the others. Its 

 condyle is characteristic, being double, and with a foramen between it and 

 the produced extremity of the posterior margin of the bone. It is slightly 

 separated distally from the third, but the remainder are in close contact. 

 The radii of the superior lobe of the caudal fin extend at least as far down as 

 near the end of the third haemal spine from below. The structure of these 

 parts in the P. molossus is as in the P. thaumas, so far as preserved. 



An outline-restoration of this genus is given on Plate LV. 



The species of this genus may be distinguished as follows:. 

 a. Two premaxillary teeth : 



Maxillary arch thin, deep, with narrow anterior condyle; large 

 maxillary teeth live ; third mandibular tooth large, behind a 

 cross-groove P. molossus. 



Maxillary large teeth three; third mandibular small, without cross- 



, groove in front of it P. thaumas. 



aa. Three to five premaxillary teeth : 



Maxillary arch stout, deep, with heavy anterior condyle ; larger 



teeth five P. lestrio. 



Maxillary arch thick and shallow ; larger teeth five P. mudgei. 



aaa. Premaxillary teeth unknown : 



Maxillary bone deeply concave ; small P. arcuatus. 



1 See Plate XLII, fig. 4. 



