52 NEW YOilK STATB MUSEUM 



to be fully justified in adopting Angelin's subdivisions in 

 preference of the oldei* ones of Forbes, on the very reasonable 

 ground that the section represented by the original name of the 

 author should correspond as nearly as possible to that author's 

 definition. These subdivisions have also been adopted by other 

 Eui\)peau wri^ters on lower Siluric trilobites. A. W. \'ogdes 

 arranges the genus in three sections, as follows: 1) Brevifrontes, 

 type A ni p y x ii u d u s Murchii<on; 2) Tvongifro-ntes, type 

 A m p y X n a s u t u s Dahnan ; 3) Lonchodonias, type L o n - 

 c h o d o ni as do m a s t u s Angt'liu. 



It is evident that the species from Rysedorph hill falls under 

 the section Ix>nclu)di)nuis of Angelin and Vogdes. 



One pygidium (pl. 3, fig. 30) proved by its mode of preservation 

 to be of exceptional interest. The crust of the axis is so faviu-ably 

 broken away that a perfect cast of the inferior surface is ex- 

 posed. This exhibits, rising from the perfectly smooth surface 

 of the matrix, two series, of paired elevations which must, corre- 

 spond to impressions on the under surface of the crust. The 

 outer series consists of the larger elevations, which near the 

 anterior margin are round, largest, but also least projecting, 

 while toward the posterior end of the axis they become smaller, 

 transversely oval, being directed obliquely outward and forward, 

 and more elevated and distinct. Alternating with this series is 

 another of small, round tubercles, and within this lies still a 

 third series of small tubercles, one always lying opposite the sec- 

 ond, fourth, sixth, etc. of the inner second series. Along the 

 median line runs a slightly elevated ridge corresponding to a 

 median furrow of the pygidium. In another pygidium, in which 

 the removal of the crust brought to light a similar system of 

 elevations, eight of these could be counted in the exterior series. 

 On the anterior part of the axis the annulation is indicated by a 

 few shallow transverse grooves. These fall between the inner 

 tubercles, and hence the elevations correspond with the annula- 

 tion and original segmentation of the pygidium. 



A series of tubercles similar to the exterior series has been 

 described and figured by Nicholson and Etheridgei from the 



1 Monograph Sll. foJW. Glrvan dlatrlct lo Ayrshire. 1880. Fasciculus 2, pl. 13, fl«. 8. 



