204 



PORTHEUS ARCUATUS, Cope. 



The smallest species of the genus, equaling Ichthyodecles ctenodon in 

 size, is represented by incomplete maxillary and palatine bones, and perhaps 

 by accompanying vertebrae and other pieces. Apart from its small size, 

 this species may be known by the compressed and concave alveolar border 

 behind and below the posterior maxillary condyle, and the very small size 

 of the teeth which protrude from its subacute edge. The superior border 

 behind this above-mentioned condyle is oblique, and its anterior border an 

 acute edge. The interior face of the maxillary is convex • the exterior 

 plane, and anteriorly dotted with radiating lines of pore-like impressions. 



This species is less nearly related to the precediug species of Portheus 



than they are to each other. 



Measurements. 



M. 



Depth of tbe maxillary behind the second condyle 0. 025 



Thickness of the maxillary behind tbe second condyle 0. 004 



Width of tbe second condyle 0. 007 



Length of tbe articular segment of tbe palatine 0. 010 



From the yellow chalk of the Solomon River, Kansas. Found by 

 Prof. B. F. Mudge. 



OBSERVATIONS OX THE. PECTORAL AND VENTRAL SPINES OF THE GENUS 



PORTHEUS. 



As already stated, the fin-rays of this genus are readily separated into 

 their constituent halves. The superior elements of the pectoral fin are 

 laminiform and concave on the side in contact with the inferior halves, 

 which they somewhat exceed in width. The inferior halves are massive, and 

 exhibit a strong superior rabbet on the posterior margin for an overlapping 

 border of the second ray. On the anterior margin is a more shallow rabbet, 

 which sunn disappears, which is covered by the superior lamina. The 

 posterior rabbet also disappears at a point varying with the species. 

 The superior lamina bears the cotylus and adjacent hook, which embrace 

 the superior scapular face! ; the inferior half supports the facets which 

 correspond to the inferior two of the scapula. The front of the pectoral 

 spine is sharp-edged, forming a thin blade, hardened by a deposit of dense 

 bone, which is transversely roughened. The blade is the edge of the supe- 

 rior lamina', which extends beyond the equally acute border of the inferior 

 half, the latter fitting closely to the concave inferior face of the former. 

 Both faces of the spine are covered with a dense layer of bone, which is 

 marked with delicate longitudinal grooves; and, when the superficial layers 

 are broken away, the deeper ones are found to be grooved in the same 



