ASAPHUS. 145 



AsAPHUS, Brongniart, 1822. 



Of so large a genus mucli might be said, and with advantage too, if our space 

 permitted us to discuss largely its affinities. The diagram, however, on page 124, will give 

 at a glance the relation which the chief subgenera of AsapJu/.s bear to one another, and to 

 the neighbouring groups, Ogygides and lllanides ; and we can more easily refer to these 

 affinities when the subgenera themselves have been defined. The genus is as remarkable 

 among the smooth Trilobites of our right-hand division (see Preface, p. 2), as Phacojjs is 

 of the left-hand group.^ There is the same perfection of organisation and compactness of 

 character, the same wide limits of variation within the genus, and as great importance 

 numerically. While however Phacops and its subdivisions are more characteristic of the 

 Upper Silurian rocks, AsapJius is strictly Lower Silurian, — scarcely ranging below, and 

 never above that horizon. The Phacopida in the higher groups are remarkably convex ; 

 the AsapJii expanded. The Phacopidm have, as a rule, the segments both of head and tail 

 well marked out ; i\\QAsaphi have them obliterated in the same portions. The PhacopidcB 

 are ornamented with a tubercular or granular coat ; the genus we are describing has a 

 smooth or only lineated one, a distinction seen and ably noted by Dr. Burmeister, who, 

 however, attached too much value to the character, since both kinds of ornament coexist 

 in several genera. The lenses of the great eyes in Phacops are very large — the equally 

 prominent eyes of Asaphus have them very small, — and so of several other contrasting 

 characters ; and while the chief character of all — the relative possession of eight and eleven 

 body-rings — keeps the two groups widely apart, they are no less sundered by the course of 

 the facial suture beneath the eye, which in all the Asaphida is to the margin behind, and 

 in all PhacopidcB to the outer margin. 



For size, only the large Paradoxides among the Olenoid group (see Preface) can match 

 the AsapJii, which include, if not quite the longest, certainly the bulkiest forms of 

 Trilobites, and indeed, so many of these, as to make size an important character of the 

 genus. Few of the species are less than three inches, many of them nine or ten in 

 length, and a few range beyond a foot in extreme measure. 



The subgenera include several extreme forms, but the essential characters of this large 

 genus seem to be as follows : — 



Form oval, without spines or tubercles on the surface, with the head and tail nearly 



^ It might, perhaps, be convenient to designate the Trilobites of the left-hand branch — which have a 

 \» ell-developed thorax, and a relativeh small pygidium of few segments, Micropyyini, and those on the right- 

 hand Macropygini. But I do not think the terms are needed. 

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