VEETEBEATES. 259 



GENUS PMGEACANTHUS, St. J. and W. 

 PNIGEACANTHUS TRIGONALIS, St. J. and W. 



PI. XXIV. Fig. 4. 



Dormal defense as seen from the side trigonal in outline, laterally 

 much compressed, walls thin, terminating in an obtuse apex pro- 

 jecting backward about 10 past a point vertical to the postero- 

 inferior angle, exceedingly attenuated inferiorly with signs of a plain 

 marginal border that was buried in the integument. Anterior edge 

 gently arched, slightly curved toward the inferior angle, rounded 

 into the gently convex lateral surfaces; posterior border apparently 

 a little more compressed, gently concave vertically, and studded 

 with relatively strong, irregular, rudely sculptured tubercles. The 

 anterior border is armed with large irregular vertically elongate 

 depressed tubercles, which seem to lie immediately on the left flank 

 of the border, forming a nearly continuous undulating ridge. The 

 tubercles, so far as may be determined by the specimen, are mark- 

 edly asymmetrical, the slope facing the anterior edge being abrupt 

 and obscurely vertically sculptured, the opposite side presenting a 

 wider gentle declivity. The lateral surfaces show more or less dis- 

 tinct longitudinal costse, irregularly striato-punctate, and studded 

 with irregular stellate tubercles whose apices are directed upward ; 

 the tubercles, as seen in the left side, vary in size and present a 

 rude sort of transverse disposition, especially toward the anterior 

 border where occasional large tubercles occur, but irregularly dis- 

 persed as relates to the longitudinal costae. The inner surface of 

 the lateral walls is smooth with a fine wavy striato-punctate struc- 

 ture. The spine may not show the perfect outline of the inferior 

 margin, but the preserved portion in the flattened condition of the 

 specimen measures about 25 millimetres from the antero to the 

 postero-inferior angle ; height of posterior border, 20 mm. ; ditto of 

 anterior border at least 33 mm. 



The collection of Mr. Alexander Butters contains the unique and 

 nearly entire example upon which the above description is based. 

 The specimen is embedded in a limestone matrix in such manner 

 as to reveal only the one side, and part of this is broken away 

 exposing a corresponding area of the inner side of the opposite wall, 

 the thin inferior portions of the walls being brought into close con- 

 tact by pressure. The specimen is unquestionably closely allied to 



