Dr.F. R. Cowper Reed — The genus Homalonotus. 821 



head-shield well within the genal angles; so that there can be no 

 question of direct genetic affinities, but only of homceomorphic 

 development. 



Classification. 



The species of Homalonotus may be grouped together into several 

 sections or subgenera on the strength of the character and course of 

 the facial sutures, the development of the epistome and doublure, 

 the degree of trilobation of the thorax, and the shape, trilobation, 

 and segmentation of the pygidium. Tliese characters are variously 

 combined, but on the whole two large divisions may be recognized 

 so far as pygidial characters alone are concerned, one of which is 

 marked by rounded and the other by pointed pygidia. In the 

 earliest members of the genus the pygidia are short, rounded, and 

 composed of few segments, and the trilobation is well marked ; in 

 some of the Devonian species the rounded form again prevails, but 

 there is a larger number of segments, and the trilobation is more or 

 less lost. In the Silurian and most of the Devonian species the 

 pointed elongated pygidium, composed of many segments, with or 

 without distinct trilobation, is found conspicuously developed. 



Many of the groups or sections have already received names from 

 various authors, but some of these groups are not homogeneous and 

 require subdivision, as the foregoing remarks have indicated. 

 Whether these groups are of subgeueric or only lesser rank may be 

 a matter of opinion ; but in the following list the groups appear to 

 possess combinations of characters of morphological importance, and 

 it will be observed that the groups also have a stratigraphical relation 

 or limitation, and therefore suggest phylogenetic significance. We 

 may therefore maintain that they are not artificial assemblages of 

 species, but correspond to certain natural divisions of the genus. 

 In some cases it is unfortunate that the species are only imperfectly 

 known, or .that specific names have been attached to mere fragments 

 of individuals, or that disconnected portions of doubtful association 

 have been brought together under the same specific designation. 

 Eut these are minor defects which are unavoidable, and subsequent 

 work may remedy them. 



Subgenera. 



1. Eohomalonotus, nom. prop. { — Brongniartia (pars), Salter, mom 

 Leach, nee Eaton). 



Head-shield transverse, rounded, more or less semicircular. Facial 

 sutures uniting close to anterior margin or on margin in regular 

 wide curve, and posteriorly cutting lateral margins slightly in front 

 of genal angles. Pre-glabellar area wide ; pre-sutural band very 

 narrow or wanting. Thorax with well-marked trilobation ; axis 

 not wider than pleural portions. Pygidium short, broad, expanded, 

 composed of few segments (six to eight); axis distinct; pleurae 

 continued to edge or nearly to it; doublure vertical or steeplj' inclined 

 at sides, simple, of nearly uniform width all round or narrowing 

 slightly posteriorly. 



DECADE VI. — VOL. V.— NO. VII. 21 



