252 THE CHICAGO ACADEMY OF SCIENCES. 



ally, especially opposite the eyes; the outer area of the cheeks is 

 moderately convex, with a raised marginal border, which ex- 

 tends from the bases of the genal spines entirely around the 

 lateral and anterior margins of the cephalon and which bears 

 a fringe-like row of^ spines directed almost vertically down- 

 ward from the margin; these spines are about twelve in num- 

 ber on each side, the longest ones being directly in front of the 

 bases of the genal spines, the succeeding ones becoming regu- 

 larly smaller until the most anterior ones in front of the anterior 

 extremities of the ocular ridges are minute. Thorax and py- 

 gidium not known. 



The dimensions of a nearly perfect cephalon are: length 

 exclusive of the posterior median spine 10 mm,, length, in- 

 cluding the spine, 16.5 mm., width at base of genal spines 21 

 mm., width at extremities of genal spines 28 mm., width of 

 glabella 10.5 mm., distance between the eyes 15 mm., con- 

 vexity 8 mm. 



Remarks. The specimen upon which the description of 

 this species is based is included in the Van Home Collection in 

 the Walker Museum. In the illustration of the species on plate 

 23, the occipital and genal spines are drawn in outline, but in the 

 specimen, 'while the casts of the spines themselves are broken 

 off as shown in the drawing, their impressions in the matrix 

 are fully preserved so that the outlines shown represent the 

 exact form of the spines. The species is quite distinct from any 

 other American member of the genus, but approaches most 

 closely to the form identified by Van Ingen as A . quinquespinosa 

 Salter-Lake, from the St. Clair limestone of Arkansas. The 

 glabella of A. vanhornei is much like that of the Arkansas 

 specimen, but the occipital spine is much more elongate and 

 more slender and it lacks the four additional protuberances 

 upon the posterior margin of the occipital segment. The mar- 

 ginal fringe of short spines in A. vanhornei is peculiar, in that 

 the spines are directed ventrally in such a manner as to be 

 wholly invisible in a dorsal view of the cephalon, only being 

 seen in a lateral or anterior view of the head; other species 

 with a similar marginal fringe of spines usually have them 

 directed more or less laterally so that they may be clearly seen 

 in a direct dorsal view of the cephalon. 



Locality. Bridgeport. 

 Genus 12. ODONTOPLEURA Emmrich, 1839. 



The members of this genus are essentially like those ,of 

 Addas pis, except in the ornamentation of the occipital segment 



