36 PROCEEDINGS OF THE NATIONAL MUSEUM vol.76 



outer edges more nearly longitudinal in direction. Though readily 

 distinguished, it yet appears entirely probable that T . telUcoensis 

 is a well modified descendant of T. hipu7ictatus, which is a common 

 fossil in the Whitesburg limestone and, so far as known, confined to it. 



The cranidium might be compared with those of various other 

 species, notably T. hircinus^ T. mobergi^ and T. prattensis^ in all 

 of which the surface shows raised and more or less anastomosing 

 lines that are characteristic of the T. hipunctatus section of the 

 genus. However, it hardly seems worth while to say more than that 

 none of the mentioned species is quite like the present. 



The free cheeks are remarkable for two reasons; first, the great 

 size and relatively coarse facetting of the eyes and, second, the in- 

 variable presence of two large diverging spines, both springing from 

 the outer side of the narrow rim, one at a point near the middle of 

 the length of the eye, the other a short distance behind it. The eye 

 facets are arranged, as usual, in quincunx, making diagonally inter- 

 secting and transverse rows with 13 or 14 of the latter sufficing to 

 cover the highest part. The outer rim is sharply separated from the 

 eye by a deep groove, very narrow in front, wider behind, and widest 

 in the middle third which bears the two large spines. The two 

 figured free cheeks show considerable difference in the form and width 

 of the spine-bearing part, and it is quite possible that the larger of 

 the two belongs to another species — perhaps to T. hircinus. 



Two very slightly differing kinds of pygidia were found with the 

 foregoing cranidia and free cheeks. They differ mainly in one being 

 relatively shorter or wider than the other. Most of the greater width 

 of the former is added to the concave and yet verj^^ narrow and ob- 

 scurely defined pleural areas and border. Both kinds have a small 

 triangular flat posterior spine, but this is a little shorter in the wider 

 form, and the edge of the pygidium turns laterally from the spine 

 more abruptly than in the narrower kind. In consequence the outline 

 of the pygidium as a whole is more regularly triangular in the 

 narrower form than in the wider one. Specimens of the cranidium 

 of T. telUcoensis being much more abundant in the rock than are the 

 parts of associated species, it is thought likely that its pygidia also 

 would occur oftener. Hence, the narrower kind, of which eight speci- 

 mens were found whereas only two examples of the wider form were 

 observed, is referred to this species. The other may belong to eithei 

 T. hircinus or T. traiisversus, with the probabilities favoring the 

 former. 



Only six or seven kinds of pygidia referable to species of this genus 

 have been detected in American deposits. The first of these occurred 

 with the cranidia of T. mysticensis. It has three blunt marginal 

 spines, two of them merely sharp angles, and three axial segments, 

 the anterior of which carries a single low median node. The second 



