4 



BRITISH FOSSILS. 



American authors, as of enormous size. The cast which is com- 

 monly sold for lecture purposes indicates a form 21 inches long ; 

 but there is no pretence for making it more than two-thirds this 

 length. AsapJms gigas is not one of the largest of Trilobites. 



Locality and Geological Position. — Caradoc of Tyrone (Port- 

 lock). The species ranges from Canada to Tennessee, and it is rather 

 remarkable that it should abound in N. Ireland without reaching 

 further to the eastward. Some other American species appear 

 to range to Ireland, but are not otherwise members of the British 

 Silurian fauna. 



Other British Species of the Section Isotelus. 



2. /. rectif rons, Portlock, Geol. Report, 1843, pi. 9, fig. 1 a, b ; 

 also pi. 8, figs. 2, S, 7, only referred to under /. planus. , 

 These belong to the head. /. arcuatus, ib., pi. 9, figs. 2, 3 

 (tail of same species). /. intermedins, ib., pi. 9, fig. 5. 

 Head semicircular convex ; the angles rounded, and showing 

 the characteristic pit for the pleuri© some distance above the angle. 

 Glabella between the eyes about equal in width to the cheeks. 

 Ej^es large, placed much behind the middle of the head, and very 

 much curved. Eye-line straight and directed outwards above the 

 eye to the front margin, along which it runs. Beneath the eye it 

 runs outwards, nearly parallel to the posterior margin. In the 

 front of the head there is no vertical suture, the front being striate 

 and showing rather a narrow base for the attachment of the 

 labrum. 



The tail and body Portlock called I. arcuatus. The body seg- 

 ments have the axis broader than the pleurae, which have the 

 fulcrum close in, and are bent back from it and rounded at the 

 ends. The tail is wider than a semicircle ; the upper angles are 

 much bent down for the facet. The axis is marked out at its origin 

 by two rather deep impressions, and is here rather wider than the 

 side lobes. Thence it is not indicated, except by a slight promi- 

 nence at its apex, which reaches to three -fourths the length of the 

 tail. A broad shallow furrow beneath the fulcrum is all the marking 

 that shows on the smooth convex sides. 



Incurved portion narrow, concave ; its edge not indented by the 

 point of the axis ; strongly lineate, the lines abutting sharply against 

 the margin. 



I. intermedius is too like the species just mentioned to be 

 catalogued as distinct. It is much pressed out of shape and 



