234 



rings of the thorax. The thoracic ring is narrow, for one is equal in length 

 to the breadth of five rings. 



HYPOSTOME. 



This for A. Homfrayi seems very imperfectly known. A good example 

 of the Cape Breton variety has the following characters : 



Hypostome of Hypostome 16x17 mm. main lobe 11 x 11. No anterior wing or dou- 

 bleur attachment was observed. 



Nearly circular, though wider towards the back than the front. It 

 has a moderately arched oval body, with a broad convex border, wider 

 towards the back. The main body of the hypostome is divided by a pair 

 of diagonal furrows that impress each outward third about two-thirds 

 from the front. Immediately behind these furrows are the maculae- sharp, 

 narrow ridges, raised above the general level of the hypostome ; no occu- 

 lar facets are visible, but there are several small, obscure pits along the 

 ridge. Were the hypostome in place beneath the glabella, the macula 

 would be opposite the eyelobe of the cephalic shield, but nearer the 

 axialine of the body. The furrow within the border is depressed at 

 the back and bordered by a narrow upturned flange, but there is no 

 emargination, nor does the border project backward in a pair of forks. 

 The hypostome is highest in the middle of the main lobe, and the convex 

 border is bent down in the middle, where it is broadest. 



DEVELOPMENT DURING GROWTH. Young, 2 x 1 mm. 



Development This larval form is interesting as a connecting link between several 

 genera of the Asaphidse. It may be said to antedate the development of 

 the generic characters. At this stage the carapace had no flattened 

 borders, and the head shield especially was strongly bent down in front 

 and at the sides. The back of the glabella is very distinctly marked 

 out, and here the head-shield is strongly trilobed. About the middle 

 of the glabella, on the inside of the shield a flaring ridge, that appears to 

 be the back part of the eyelobe, runs out on each side from the glabella, 

 and fades away on the surface of the test. At this stage no moveable 

 cheek had been detached, but the genal corner of the shield is somewhat 

 extended into a short point. There are indications of several somites in 

 the head shield ; first the neck ring and posterior marginal fold, then a 

 pair of somites indicated by incipient furrows on the sides of the gla- 

 bella, then the ocular segment. 



The thorax, at this stage, possessed two joints, with rounded rings and 

 pleurae. 



