CRUSTACEA 



149 



minute segmental furrows at each side of the narrow base ; 

 thoracic segments five, pleurae of each side twice the width of 

 the axal lobe ; pygidium very obtusely and regularly rounded, 

 four times wider than long, axis with about seventeen minute 

 segmental furrows, sides with about eight. Length of entire 

 animal 3^ lines. 



This rare species is most allied to the A. parvulus (Forb.) and 

 the A. nasutus (Dal.), from both which the perfect animal is 

 easily known by its transversely oval form ; the regular curvature 

 and great width of the cephalic and pygidial shields easily distin- 

 guish those parts when found separate ; the latter agrees nearly 

 in form with that of the A. parvulus, from which it differs equally 

 with the former in all the other characters of cephalic shield, &c. 

 Rare in the black Wenlock shale three miles north of Builth. 

 [Col. University of Cambridge.) 



Tretaspis (M^Coy), n. g. 



Gen. Char. General characters of Trinucleus, but having only five 

 body-rings, the base of the glabella having 

 two short segmental furrows at each side, 

 and the cheeks being traversed diagonally by 

 a nearly straight eye-line, extending on each 

 side from the junction of the cheeks and 

 glabella in front, towards the lateral angles CephalSieTd'show- 

 cutting the posterior margin a little within ing the eyes and 

 the angles, and usually exhibiting a small diagonal facial su- 

 ocular (?) tubercle in the middle. Types i^"»es. 

 of the genus Trinucleus seticornis (His.) sp., T. Bucklandi 

 (Bar.), &c. 



In my ' Synopsis of the Silurian Fossils of Ireland ' I pointed 

 out the course of the eye-line in this genus, which separates it 

 at once from Trinucleus, and renders it probable that the small 

 tubercle in the middle of the cheeks in the T. seticornis, T. fim- 

 briatus, &c. are true eyes. The furrows at the base of the gla- 

 bella also are distinctive for the genus*. 



Trinucleus gibbifrons (M^Coy). 

 Sp, Char. Cephalic shield nearly semicircular, length rather more 



* The statement of Mr. Salter (Mem. of the Geol. Snrv. vol. ii, pt. 1. 

 p. 335), speaking of Hawle and Covda's work, that " Tetrapsellium is distin- 

 guished from Trinucleus solely by a swelling in the axal fuiTow of the head ; 

 it is almost identical else with T. seticornis " — might mislead the English 

 reader with the idea that the present genus was identical with Teirapsellium ; 

 the fact is however, in his stricture on the Bohemian authors, Mr. Salter 

 seems to have overlooked the grand character of their genus, namely its 

 having but four body-rings (" vier Leibringe," H. & C. Monog. p. 42. 

 8th line) ; it agrees otherwise with the common type of Trinucleus. 



