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



occasionally spinous. Internally the cheeks have at their base 

 a broad flat space next the glabella". 



Raymond (op. cit., 1913, p. 724) die^nes Homalonotus as follows: 

 " Axial lobe wide, cephalon short and trilobate in front, cheeks 

 forming high mounds crowned by tlie eyes." Apparently he had in 

 his mindiZ". Knighti, and at any rate this definition would not include 

 any of the Ordovician species, and none of them can be put in 

 Trimerics or Dipleiira, which are the only other Homalonotid genera 

 which he quotes. 



Woodward,' in 1903, practically repeated Salter's definition in 

 slightly different phraseology, saying: "The peculiar trilobation of 

 the body-rings, so conspicuous in most genera, is very indistinct 

 in Somalonotus, especially in the thoracic segments, although m 

 some species it is better marked in the pygidium. The shape of the 

 body is elongate, convex, with steep sides and a very broad axis, 

 scarcely distinguished from the pleuras. There are thirteen body- 

 rings, deeply grooved, and the fulcrum is close to the axis in most of 

 the species. The head is triangular, with an obscure quadrate 

 glabella slightly lobed and a quadrate labrum ; the surface of the 

 body is scabrous, occasionally spinous. The pygidium is generally 

 narrow and pointed, except in a few species which have a more 

 rounded contour." He goes on to say that "of the twenty species 

 recorded, by far the larger number are from the Silurian". It will 

 be shown in this paper that the climax of the development of the 

 genus is in tlie Devonian period, where the number of species and 

 the diversity of types are greatest. 



2. Tnmerus, Green, 1832. 



The type of this section or subgenus is the well-known JT. delphino- 

 cephalus. Green, ^ first described from the Niagara Limestone of New 

 York and subsequently recognized in the Wenlock Limestone of 

 Dudley (accoi-ding to Salter^ it is the Woolhope Limestone). Salter 

 (op. cit.) defined this section as folloAVS : "Elongate, convex, with 

 triangular liead; eyes not remote; a defined but obscurely lobed 

 broad glabella. Thorax slightly lobed ; tail many ribbed, pointed, 

 often acuminate." Raymond (op. cit., p. 724) merely states that the 

 "cephalon is longer than in the preceding [i.e. Homalonotus], not 

 trilobate in front, free cheeks narrow". There seems to be some 

 difference in the British and American specimens as regards the 

 glabella, for Hall^ does not mention or figure any glabellar lobes or 

 furrows in his description of the Niagaran types, while in most of the 

 Dudley specimens two pairs of more or less faint subcircular lateral 

 swellings, not touching the axial furrows, are present on the 

 posterior half of the glabella and seem to represent the two posterior 

 pairs of lateral lobes, and there is also a weak trace of the first 

 lateral furrows. 



The peculiar subcircular areas on each side of the base of the 



^ Woodward, Geol. Mag., Dee. IV, Vol. X, p. 28, 1903. 



" Green, Monthly Amer. Journ. GeoL, vol. i, p. 559, pi. 0, fig. 1, 1832. 



^ Salter, op. cit., p. 115. 



* Hall, Palseont. N.Y., vol. ii, p. 309, pi. Ixviii, figs. 1-14. 



