220 NEW YORK STATE MUSEUM 



Genus aechmina Jones & Holl 



[Ety. : ar/!J.r'j, point of a spear] 



{An. and mag. nat. hist. ser. 4. 3:217) 



Carapace with thick valves, straight at hinge line, rounded at the- 

 ends, and convex at the ventral border. Surface drawn out into a 

 broad-based and sharp pointed hollow cone, which either involves 

 the whole surface, or rises from the postero-dorsal or centro-dorsal 

 region. 



Aechmina spinosa (Hall) (Fig. 152). Cytherina spinosa 

 Hall (1852. Pal. N. Y. 2:317, pi. 67) 



Distinguishing characters. Strong oblique spine^. 

 thick and hollow at the base, either elongate or 

 short; pointed upward, outward and forward, and 

 sometimes slightly bent; valves thickened on the 

 free border by a raised, rounded but irregular mar- 

 gin; area at base of spine hollow and smooth; 

 raised margin sometimes punctate; spine often long 

 and projecting beyond the upper margin of the 

 valve. 



Found in weathered Rochester shale on the talus 

 Fig. 152 Aechmina ^^ ^^'^^ Romc, Watcrtown and Ogdensburg railroad 

 ^'^After^jones^^^^^^^ cut abovc Lcwistou. The valves are often im- 

 bedded in the shale with the inner concave surfaces exposed. Alsa^ 

 found at Lockport (Hall). 



Order TRILOBITA Burmeister 



The trilobites are extinct Crustacea, wholly confined to the Paleo- 

 zoic seas. The body was covered with a carapace longitudinally 

 divisible into three parts. The anterior portion comprises the head- 

 shield, or ccphalon, which is usually semicircular, with a straight pos- 

 terior border. The central of the three cephalic lobes is the glabella^ 

 which is the most prominent part of the cephalon. It is of varying 

 outline, and more or less divided by transverse furrows or pairs of 

 furrows. The last furrow is the occipital furrozv, and delimits the 

 occipital rm^, which is just anterior to the first segment of the thorax. 

 On either side of the glabella is a pair of cheeks, divided by the 

 facial siitnre into fixed cheeks (those next to the glabella) and free 

 cheeks (the outermost portion). The latter are often prolonged inta- 



