84 SYSTEMATIC ARRANGEMENT 



Genus 12. — Homalonotus, König, (Trimerus, Green, Murchison ; Dipleura, Green.) 



Cephalic shield hyperbolic, the anterior angle rather acute, the lateral margins gently 

 arched, the posterior margin tolerably straight, without the angles being extended back- 

 wards ; the whole nppcr surface gently arched, but the region at the external margin 

 and the border at the posterior margin rather flatly expanded ; the latter separated by a 

 furrow, slightly arched. The glabella undivided, broader towards the posterior than at the 

 anterior part, and occupying about two thirds of the posterior margin. It then conti'acts 

 a little, and assumes a round shape towards the anterior part ; without lateral lobes and 

 sutures, but there is a trace of a furrow, which issued from the region where the e3'es are 

 situated beside the glabella. This furrow is produced backwards towards the central line, 

 and may sometimes be perceived with tolerable distinctness. Both furrows correspond 

 with the posterior furrows of the glabella of the preceding genus, and separate the anterior 

 cerebral region, the forehead, from the posterior or branchial region. 



Eyes placed near the centre of the glabella, just in the centre of the lateral parts of the 

 shield, or a little behind it, depressed, corapai'atively smaller than in Cahjniene ; usually 

 similarly excavated. 



Facial suture parallel anteriorly with the margin of the cephalic shield, but apart from 

 the latter ; acutely angular, terminating on the flat extension of the cephalic shield ; thence 

 turning in the form of an arch towards the eye, over which it forms the well-known covering 

 plate, and then bends itself with an S-shaped curvature towards the posterior lateral angle, 

 which is divided into two halves, either in the angle itself, or before the point towards the 

 outer side. 



Axis of the body thirteen-jointed, decidedly narrower towards the posterior part ; the 

 axis itself but slightly arched, owing to which the lateral lobes (the transverse diameter of 

 which is smaller than the transverse diameter of the axis) are not as strongly separated from 

 it as usual ; the posterior margin of each separate ring of the axis is produced forwards, 

 acutely angular, sometimes (in Diplriira) even rather turned up ; the anterior or articular 

 portion separated by a more or less impressed transverse furrow, proceeding from the 

 posterior part of the ring. This character does not belong to any other genus of the Trilo- 

 bites, and on that account appears to me a most important and peculiar one.* 



Caudal shield hyperbolic, longer, but much more narrow than the cephalic shield, and 



* In interpreting the fragments of tliis genus, we ought to be very particular in obserriug 

 whether the impressions of the rings originate from the upper exposed surface of the back, or from the 

 inner surface, which is turned towards the soft part of tlic animal. lu the former case the transverse 

 ftirrow, which separates the articulating portion from the ring itself, appears as a fine line, and thus it 

 has also been represented in Murchison's Figures (Tables VII and "N'lII, as far as Figs. 1, 2) ; in the 

 latter it forms a deep broad furrow, which originates from a horny process of the ring that hangs 

 downwards towards the inner part, and owing to this process being thick, it is also broadly and deeply 

 impressed into the matter inclosing it. Thus appear Mm-chisou's Figures 3 and 4 in Table VII. 

 Hence it follows that impressions differing from each other in the manner described do not indicate 

 different species, but different sides of the shell of the same species. My Figures, Table IV, Figs. 10 

 and 11, show the difference in the rings of the shells of Homalonotus and Calymene more particularly, 

 and respecting their significance I refer the reader to the explanations of the plates. 



