Dr. F. R. Cowper Reed — The genus Homalonotus. 265 



manner, including many species Avhicli differ widely from the type. 

 For this reason Salter/ in 1865, was led to suggest a new name, 

 Kcenigia, for the section characterized by H. Knighti, and its 

 characters are discussed below under that heading. Other authors 

 have not been so precise, and Hall's^ definition of Homalonotus strove 

 to be wide enough to embrace the various divergent sections or 

 groups, and has, therefore, a more extended application: "Body 

 usually large, produced, depressed above, with abruptly sloping sides. 

 Axial furrows indistinct or obsolete. Surface smooth or spinose. 

 Cephalon depressed-convex, wider than long; genal angles rounded ; 

 anterior margin somewhat produced; glabella subrectangular, smooth^ 

 or with faint lateral furrows; eyes small, situated somewhat back 

 of the middle of the shield ; the facial sutures run from the genal 

 angles over the eyes, converging towards the frontal margin, where 

 they are connected by the transverse frontal suture, thence they 

 continue to the edge of the doublure where they meet, thus inclosing 

 a small, free, subtriangular plate. Thorax composed of thirteen 

 deeply sulcate segments. Pygidium smaller than the cephalon, 

 elongate triangular, posteriorly rounded or slightly produced. The 

 axis bears usually from ten to fourteen annulations. Pleurae smooth 

 or with posteriorly sloping ribs." This definition, however, is not 

 entirely satisfactory, for it is not applicable to the early Ordovician 

 species, in which the axial furrows are distinct, and the number of 

 segments in the pygidium fewer than stated. Hall also seems to 

 regard the commissure uniting the facial sutures in front as not 

 a part of them but as a new and separate structure, and he fails to 

 remai'k that the epistomal sutures are distinct. His " subtriangular 

 plate " is the epistome or rostral shield. It should, moreover, be 

 added that the thoracic pleurae always have rounded ends, and that 

 the pygidium exhibits two main and separate types, one semicircular 

 or semi-elliptical and simply rounded, the other triangular and 

 acuminate behind. 



The point of section of the margin by the posterior branch of the 

 facial suture is also of some importance ; Hall shows it in his 

 diagrammatic woodcut as bisecting the rounded genal angle ; but this 

 is not always the case, as Koch (op. cit.) noticed and used as a basis 

 of classification. 



Salter's^ earlier definition in 1865 makes no reference to the 

 pygidial charactei's nor to the facial sutures, but it says that the 

 genus is distinguished from Calymene., " its near ally," by its want of 

 distinct trilobation, and goes on to state that '■'■Homalonotus. is elongate, 

 convex, with steep sides and a very broad axis, scarcely distinguished 

 from tlie pleurae. There are thirteen body-rings deeply grooved, and 

 the fulcrum is close to the axis in most of the species. The head 

 with an obscure quadrate glabella, slightly lobed ; a rostral shield ; 

 and a quadrate labrum (= hypostome) tuberculate and gibbous in 

 the middle and with a bilobed tip. Surface of the body scabrous, 



' Salter, Mon. Brit. Trilob., p. 106. 



^ Hall, Palffiont. New York, vol. vii, p. xxiv, 1888. 



3 Salter, Mon. Brit. Trilob., p. 104. 



