350 PALJ50NT0L0GY OF NEW- YORK. 



CRUSTACEA. 



The Crust-acea of the Lower Helderberg group are not numerous ; and 

 though the collections have been continued in the Helderberg mountains, 

 and at Schoharie and Carlisle as well as other places, through many years, 

 the specimens that have been obtained are for the most part fragmentary. 



The small number of individuals, of nearly all the species, which have 

 been found, shows that they are comparatively rare in the localities 

 examined. Two species, the Dalmania pleuroptyx and D. micrurus, occur 

 in considerable numbers ; mostly, however, preserving only the caudal 

 extremity. After these, the Phacops logani ; while the Acidaspis tubercula- 

 tus is of frequent occurrence in certain layers, but always in the condition 

 of fragments. 



The genera occurring in this group are Bronteus, one species ; Proetus, 

 one species ; Homaxonotcs, one species ; Phacops, one species ; Dalmania, 

 four species; Lichas, two species; Acidaspis, two species. 



There are, 'besides the trilobites, several species of Beyrichia and of 

 Leperuita. 



Bronteus Imrraiuli ( n. s.). 



Plate LXXIII. Fig. 1, 2, 3, 4. 

 Pygidium semielliptical ; line of articulation straight; rudiment of the 

 axis abruptly prominent, nearly twice as wide at its upper edge as the 

 length, showing two articulations. Surface around the axis nearly plane, 

 and thence sloping more abruptly, and again becoming nearly flat at 

 the margin ; marked by seven ribs on each side the median lobe, which 

 is wider than the lateral ones and gradually narrowing from the base 

 of the axis for one-fifth of its length, below which it gradually expands 

 towards the border without bifurcating ; its surface scarcely more 

 prominent than the lateral ribs, and the furrows limiting it not more 

 profound than the adjacent ones : lateral ribs narrow at their origin 

 and gradually expanding towards the margin, the upper one wider at 

 its outer extremity than either of the others. 



