from the Silurian Rocks. 161 



smaller species occurs in the black anthracite shales of Dumfries- 

 shire, which are generally believed to be of Lower Silurian age. 

 It is the 



11. Ceratiocaris aptychoides. 



(Dithyrocaris aptychoides, Salter in Quart. Geol. Journ. vol. viii. 

 pi. 21. fig. 10) a minute species. 



Locality. Duffkinnell, Dumfriesshire. Prof. Harkness. 



This would appear to belong to the Llandeilo flags ; but it is 

 possible the age of the formation may be more accurately given 

 by the fossil, and the black anthracite shales may not have the 

 antiquity they at present claim. 



On the whole, about fifteen species have been recorded of this 

 interesting genus, which combines to some extent the charac- 

 ters of Nebalia and of Limnadia. It will doubtless form the 

 type of a distinct family. 



With Ceratiocaris, in the south of Scotland, is associated 

 another and still larger form, which I will now describe. 



DlCTYOCARIS. 



More plentiful in fragments than even the Ceratiocaris, in the 

 shales and sandstones of the Lesmahago hills, is a great Crusta- 

 cean, apparently of very thin texture, whose carapace frequently 

 measures from 9 inches to a foot in length ! This rivals in size 

 the great Pterygoti, among which its remains are often found. 

 The anterior termination of the carapace has not been seen ; its 

 posterior edge is truncate, as in Ceratiocaris, and with a strong 

 marginal furrow ; and its ventral border must have been so 

 produced as to give it a subtriangular outline. Some fragments 

 of body-rings found with it may indicate its relationship to 

 Ceratiocaris, from which its simply bent (not bivalved) shield 

 distinguishes it. 



The entire surface of the carapace is marked with hexagonal 

 reticulations ^th of a line in diameter, of which the arese are 

 convex, and the bounding lines sunk on the exterior aspect. 

 This would, I think, indicate the ornament to be connected with 

 the structure of the carapace rather than to be a mere external 

 sculpturing. As no films can be obtained thick enough to 

 furnish a section for microscopic examination, the point cannot 

 be ascertained. 



There are probably two, if not more species. I describe only 

 one at present. 



DICTYOCARIS, n. g., 1860. 



Carapace ample, bent along the dorsal line, but not two-valved, 

 largely reticulate, the area of the reticulations being convex. 



