Ueport of the State Geologist^ 95 



species is not well known except from its cephalon which is notable for 

 the extraordinary pair of curved spines originating upon the 

 very broad occipital ring, making almost an entire revolution in 

 a spiral outward curve and resembling a pair of grappling irons. Mr. 

 Conrad regarded this fossil as ttie entire animal, describing it 

 as without thoracic segments but with a tail prolonged into two 

 great spines; hence the generic name, which is thus a mis- 

 nomer, but can not be impeached on such a ground. But a single 

 additional species of this well characterized group is known, the 

 Acidaspis monstrosa, Barrande, from an equivalent horizon, the 

 stage G. Fragments of the thorax known to belong to these species 

 have been figured by both Hall and Barrande and they indicate 

 structure similar to that of the Ceratocephalse generally, straight 

 pleurae, prolonged into spines which are abruptly curved backward 

 and greatly produced. Barrande also figures* a very incomplete 

 pygidium which he surmises may belong to A. monstrosa, but it has a 

 lichadiform structure and is totally unlike that figured by Professor 

 Hall f in connection with a portion of the thorax of A. hamata. The 

 original of the latter drawing is in the collection of the State Museum, 

 but the pygidium, which had become detached and been gummed in 

 place, has again become loosened and lost, a most regrettable circum- 

 stance as assiduous search in the Lower Helderberg rocks which has 

 produced many fine specimens of this fossil has failed to discover 

 another of the pygidium. We may, however, confide in the accuracy 

 of the original representation of this part, in which the pygidium 

 appears to have been of small size, semi-circular in outline, with a 

 single projecting spine on each side near the anterior margin. The 

 figures given on the accompanying plates of the American and Bohe- 

 mian species will serve to indicate the distinctive characters of this 

 group. 



In the work of Corda's already cited (1847) the name Selenopeltis J 

 was proposed, and four species described, S, Stephani, S, Buchi 

 {Odontopleura Buchi, Barrande, 1846), S. Beyrichi, and S. Humboldti. 

 Barrande demonstrated in 1852 § that all the fossils thus described 

 represented but one species, his Acidaspis Buchi. Corda's generic 

 name was well defined and fully illustrated. This species Acida^s 

 Buchi is remarkable in many respects and we have reproduced Bar- 

 rande's figure of the entire animal which is almost a copy of that 



*Vol. 1, Supplement, pi. 15, flg. 3. 



t Palaeontology of New York, vol. 3, pi. 79, flg. 19. 



t Page 34. 



§ Page 746. 



