[MATTHEW] STUDIES ON CAMBRIAN FAUNAS 45 



The furrows on the surface of the glabella, except the posterior pair, 

 are shallow undulations, and there are three pairs. The occipital ring 

 bears a small tubercle. 



The movable cheek was not known to the author of the species ; this 

 part is wide and strongly arched on the external margin. Exclusive of 

 the spine it is somewhat more than twice as long as wide, and has a 

 shallow furrow within the narrow marginal rim. The spine is short, 

 reaching across the first two pleurae only ; it is weak and often detached. 



The eye-lobe is short, being half as long as the suture in front of it 

 and one third of that behind it. With the movable cheek in place the 

 head is more than twice wider than long. 



There appears to be some variation in the number of joints in the 

 thorax ; Dr. Eoeminger enumerates from fourteen to seventeen, but the 

 lower numbers were probably found in mutilated individuals, Mr. Walcott 

 found nineteen in an example 23 mm. long ; the author has found 

 eighteen as a constant number in examples of a length of 16 mm. 

 upward to the adult size. There is a row of low tubercles along the 

 median line of the thorax. 



The pygidium is very compact ; the rachis is large in 23roportion to 

 the side lobes and extends nearly to the posterior margin ; three somites 

 are indicated by shallow furrows, and the side lobes have two pairs of 

 furrows, of which the posterior is very faint. 



Sculpture. — The surface is minutely granulated, and on the cheeks 

 has faint sinuous lines radiating from the eye-lobe, ocular fillet and front 

 of the glabella to the anterior and lateral margins of the head-shield ; 

 these lines are most distinct on the interior surface of the test. 



Size. — About 25 mm. seems to be the limit of length of the body of 

 this species. 



It is difficult to choose between Ptychopana and Solenopleura as the 

 genus for this species.^ To the former it is related by its rather shallow 

 cephalic furrows and its genal spines ; to the latter by its pleurse with 

 rounded ends, its short compact pygidium and its granulated surface. 



This species is closely like P. Kingi in form, but has a wider head- 

 shield and narrower pleurae ; in that species, according to Mr. "Walcott's 

 figure, nine segments of the thorax are equal in length to the cephalic 

 shield, in this ten joints equal the length of the cephalon. 



P. Adamsi may be distinguished from this species by its wider fixed 

 cheek, and more direct dorsal suture. 



1 Roy. Soc. Can. Trans., vol. v., p. 134. 



