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HOMALONOTUS JOHANNIS (Salter). 



(A local and rare species of Trilobite from Usk, Monmouthshire. V 



By J. W. Salteb, Esq., F.G.S., &c, 



A finer specimen than any yet collected of this fine species adorns the 

 choice and well-arranged cabinet of Dr. Harvey B. HoU, of Worcester. This 

 accurate and painstaking geologist and naturalist, for he combines both sciences 

 well, is fortunate enough, that is, diligent enough, to secure good fossils 

 wherever his hammer falls. And he permits me to figure and describe this 

 specimen as a supplement to my account of the species in the transactions 

 of the Palaeontographical Society. 



Homalonotus, as its name imports, is the least Trilobitic of all- the 

 Trilobites, Le., it has the surface of the body less divided into three lobes 

 than any other genus, certain forms of Illoenus excepted. Unlike the Dudley 

 locust ( Calymene) in every point of mere habit and appearance, it is yet ao 

 strongly related to it, that certain forms in the lowest rocks in which both 

 are found (Arenig or Skiddaw recks of Sedgwick) may pass one for the other. 

 Yet when we come to Upper Silurian times, or Devonian epochs, no two genera 

 can be more distinct. And while Calymene, beautiful as she is, is short of 

 stature, Homalonotus is bulky ; Calymene is smooth, Homalonotus often very 

 roughly sculptured. Some «f the later species (Devonian) are armed with large 

 spines along their backs, and all about their heads ; and even their tails bear 

 great boil-like spines. Usually, however, they are not so ornamented or encum- 

 bered, and a roughly granular surface is the general character. The older ones 

 (Arenig and Lower Silurian) are often considerably trUobed. All the more 

 modem ones (Upper Silurian and Devonian) are nearly free from trUobation. 

 All possess a more or less elongate and very convex form, the depth of this 

 genus contrasting strongly with the flattened, shallow forms of Ogygia, Para- 

 doxides, Asaphus, and other large forms, with which Homalonotus may be com- 

 pared in size, though widely different in character. 



Homalonotus delpkinocephalus, of Green, the common and handsome fossil of 

 the Dudley limestone is the species with which our Wenlock shale should be com- 

 pared (see plate over leaf. ) The differences are as f oUows : Both are nearly of a size, 

 full-grown specimens, measuring six inches, seldom more. But while the Dudley 

 fossil has the Gothic head and triangular tail nearly equal in size, the tail 

 piece of H. Johannis, including the terminal spine, is rather longer. One 

 species is more strongly trilobed, especially in the head, the glabella or central 

 portion (stomach) being marked out much more strongly as a trapezium, and 

 having turned sides and well marked lobes, while that of the Dudley fossil 

 is more faintly indicated in all these particulars, and has the hinder portion 

 narrower in proportion, and the lobes very faintly marked. The head too is 

 truly triangular. The thorax (or body, for it includes not only the true thorax, 

 but part of the abdomen in all trilobites) is of 13 rings, but only 10 or 11 show 



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