NO. 9 NEW CAMBRIAN TRILOBITE FAUNULE — RASETTI 37 



STIGMADISCUS GIBBOSUS Rasetti, new species 

 Plate 3, figure 4; plate 12, figures 8-13 



Available material. — About half a dozen cephala, mostly in medi- 

 ocre state of preservation. 



Description. — Cephalon equally wide and long, moderately convex, 

 semielliptical. Glabella narrowed forward in its posteriormost portion 

 from the wider occipital ring, almost parallel-sided in its medial 

 part, rapidly tapered and somewhat pointed in its anterior fourth. 

 Two posteriormost pairs of lateral furrows deep, short, pit-like ; no 

 indication of other pairs. A broad transglabellar depression is located 

 somewhat in advance of the anterior pair of lateral furrows. The 

 glabella rises in bulbous shape behind this depression and drops 

 sharply to the occipital furrow. The occipital furrow is impressed 

 at the sides in form of a pair of small pits, shallower than the glabellar 

 furrows, not connecting with the axial furrow, and is extended medi- 

 ally into a shallower, still well-impressed portion. Occipital ring 

 short, bearing a small node. Cheeks rather flat in front of the 

 glabella, not forming a definite depression. Border furrow shallow 

 and poorly defined anteriorly, gradually becoming narrower and 

 deeper toward the genal angle. Border poorly defined, somewhat 

 convex, reduced in width toward the genal angle. Posterior border 

 furrow directed outward and forward, forming a considerable angle 

 with the posterior cephalic margin and hence producing an elongate, 

 triangular posterior border which extends into a short spine located 

 very close to the genal angle. The posterior cephalic margin slopes 

 down somewhat in connecting the posterior with the lateral border. 

 Surface of test smooth. Length and width of largest cephalon 6.8 mm. 



Occurrence. — Collection cs-4. North Chatham, 



Discussion. — This form seems closely related to the type species, 

 with which it shares the pit-like glabellar furrows and the elevation 

 of the posterior portion of the glabella. In other respects there are 

 conspicuous differences, which made the writer hesitate whether the 

 two should be referred to the same genus. In Stigmadiscus gibbosus 

 the glabella is larger in proportion to the whole cephalon, the occipi- 

 tal ring is shorter and lacks a spine, the glabellar boss is defined both 

 in front and back, instead of rising gradually in the frontal part as 

 in 5*. stenometopus. Perhaps the most significant difference of all is 

 in the position of the spine on the posterior cephalic border, in the 

 present species close to the genal angle so that it may be called a 

 "genal" spine. These various differences give to Stigmadiscus gib- 



