268 Dr. F. R. Coivper Reed— The genus Homalonotus. 



The former, however, should be placed in the section Koenigia 

 according to the structure of the front margin of the head-shield, 

 which Salter failed to observe from want of well-preserved specimens. 

 But H. cylmdncus is undoubtedly a member of Trimerus so far as its 

 pygidium is concerned, and it was on this part that the species was 

 based. Salter's ' outline-sketch of a middle-shield (of which the 

 specimen cannot be traced), which he thought might belong to this 

 species, shows the facial sutures uniting in front in a much flattened 

 curve, but there is nothing in other respects to prevent its reference 

 to the same section as S. delphinoceplialiis. 



3. Dipleura, Green, 1832. 



This section or subgenus was founded on the well-known American 

 species H. Dekayi, Green, ^ from the Hamilton Group (Middle 

 Devonian). 



Salter (op. cit., p. 105) defines the section as follows: " Convex, 

 head wide, semi-oval or subtriangular, with somewhat pointed front. 

 Glabella narrow, well defined. Eyes rather remote, on gibbous 

 cheeks. Thorax slightly lobed. Tail obtuse, hardly ribbed." No 

 British representatives are recorded, but Salter refers to it the 

 Continental species S. obtusus, Sandb., and probably H. crassicauda, 

 Sandb., and H. Ahrendi, lloem. 



Raymond (op. cit.) briefly defines Diplem'a as follows: "Axial 

 lobe wide, pygidium smooth." 



HalP has given a full description of the species, and we may 

 quote his description of the course of the facial sutures : " The facial 

 sutures take their origin on the lateral margins of the doublure in 

 front of the genal angles and pass inward, parallel to the posterior 

 margin of the cephalon, to the eye, thence forward with a broad 

 curve inward to the anterior margin at the base of the prora, bending 

 thence on to, the epistomal doublure, meeting at its inferior margin. 

 The branches of the facial suture are united on the upper surface of 

 the prora by a straight transverse frontal suture, thus leaving a free 

 median plate upon the epistoma, which is elongate-subtriangular in 

 outline, attenuate at the apex, and recurved at the base, which 

 forms the anterior portion of the prora." The connecting "frontal 

 suture " here described, judging from the manner in which the 

 facial sutures bend inwards and are connected in front of the 

 glabella in H. noticus, Clarke, and other species, must be regarded as 

 merely the anterior deflected part of the facial sutures, while the 

 so-called continuations of the facial sutures over the anterior edge 

 which bound the epistomal plate laterally on the inferior surface of 

 the head-shield must correspond with the epistomal sutures of other 

 species. 



Here, as in //. ornaius, S. rhenatius, and other Devonian species, 

 the flattening of the anterior curve of the anterior conjoint portion 

 of the facial sutures gives a spurious appearance of an abnormal 



^ Salter, op. cit., p. 117, fig. 28. 



- Green, Mon. Trilob. N. Amer., 1832, p. 79, pi. i, figs. 8, 9. 

 ^ Hall, Palasont. New York, vol. vii, p. 7, pi. ii, figs. 1-11 ; pi. iii, figs. 1-5 ; 

 pi. iv, figs. 1-6 ; pi. V, figs. 1-10, 1888. 



