25O THE CHICAGO ACADEMY OF SCIENCES. 



The dimensions of the most complete head that has been 

 studied are approximately: length 28.5 mm., width between 

 the eyes 44 mm., greatest width of median glabellar lobe 20 

 mm., greatest width of glabella 34 mm. The approximate 

 dimensions of an incomplete pygidium are: length 120 mm., 

 width 76 mm., width of axis at anterior margin 26 mm. 



Remarks. This species is best known from the pygidium, 

 several examples of which, from the Van Home Collection, have 

 been under observation. The heads are less numerous than 

 the pygidia and are even less perfectly preserved, only two 

 examples have been seen , both upon a single specimen of lime- 

 stone about 12x15 cm. in size where they are associated with 

 two fairly well preserved pygidia, so that the correlation of the 

 head and pygidium of the species is satisfactorily established. 

 The large hypostome illustrated on plate xxn., figure 14, is as- 

 sociated with the parts of this species and is believed to belong 

 here. 



Locality. Hawthorn. 



Family 6. ACIDASPIDAE Barrande. 



"Dorsal shield spinose. Cephalon transversely semi- 

 elliptical, quadrate or trapezoidal; genal angles spiniform. 

 Glabella with one large median axial lobe and two or three 

 lateral lobes. Free cheeks large, separate. Sutures extending 

 from just within the genal angles abruptly inward to the eyes, 

 and then forward, cutting the anterior margin each side of the 

 glabella. Eyes small, often prominent. Thorax of eight to 

 ten segments, with ridged pleura extended into hollow ^spines. 

 Pygidium usually small, with spinous margin." Beecher, 

 Zittel-Eastman Text Book of Paleontology, p. 633. 



Genus n. ACIDASPIS Murch., 1839. 



Entire body sub-elliptical in outline, strongly trilobate, 

 the margin of the head, thorax and pygidium bordered with 

 spines. Cephalon usually sub-crescentiform in outline, some- 

 times sub-truncate in front, the genal angles produced into 

 spines. Median lobe of the glabella elongate, extending from 

 near the anterior margin to the occipital furrow, with two or 

 three pairs of lateral lobes which are successively larger from 

 the anterior to the posterior, the occipital segment bearing a 

 single median spine directed posteriorly. Thorax with nine or 

 ten segments. Pygidium with two or three annulations. 



