DITHYROCARIS ORBICULARIS. 169 



Specific Characters. — This represents the specimen which was described but 

 not figured by General Portlock. The outline of its left moiety is almost semi- 

 circular, and the probably similar edge of the right valve may be regarded as 

 conterminous with the right-hand broken edge of the specimen. This is in 

 accordance with Portlock's view also. Its postero-ventral spine is still traceable 

 in places. From this, across to the opposite angle (of left valve), is the obscure 

 posterior edge of the carapace, with faint traces of the medio-dorsal and postero- 

 ventral spines. 



From the left edge the first ridge is 7 mm. From the first to the second 

 ridge is 5 mm. From the second ridge to the right-hand edge of the specimen is 

 10 mm. Taking the first ridge for a mesolateral, and the second ridge for the 

 mid-dorsal (and evidently so regarded by Portlock), the width of the valve is 

 nearly 12 mm., and its length (and that of the carapace) is about 18 mm. (with- 

 out the spines). The whole width of the carapace was probably about 24 mm. 



The right valve is unfortunately hidden, and perhaps broken up under the 

 matrix on that side, its hinder spine only remaining in evidence. The anterior 

 edge, like that of the left valve, is lost. Besides the two prominent crenulated 

 rugose ridges, there is a small (cephalic ?) ridge between the front end of the left 

 mesolateral ridge and the ventral edge, and some displaced fragments of similar 

 but thinner ridges in the posterior region. The left valve, besides having an 

 obscure trace of its posterior spine, is characterised by its fringed edge being con- 

 tinuous throughout. 



General Portlock particularly points out the differences between the shape 'of 

 this form and that of his D. Colei. " The length of the single valve '6 inch, 

 breadth "4 inch. The lateral line prolonged, would form a chord nearly as long 

 as the axis, and the versed sine would be "25 inch, or more than one-third of the 

 chord, a proportion very different from that of the preceding species." See above,, 

 page 167. 



A somewhat similar orbicular carapace may be noticed in Lepidurus bilobatus, 

 Packard, ' North American Phyllopods,' 1883, p. 318, pi. xv, fig. 3. 



This interesting and rare Irish fossil was obtained by General Portlock and his 

 colleagues on the Geological Survey, in the Lower Carboniferous Shale at Bally- 

 nascreen, on the Whitewater River, Derry. The shale is black, calcareous, 

 containing a few small obscure Ostracods. 



Sir Frederick M'Coy stated in his ' Carb. Foss. Ireland,' p. 163, that — 



" I have only seen a few fragments probably of this species along with the 

 last [D. Colei] ; it is distinguished by its nearly circular outline, and its tubercu- 

 lated lateral and mesial ridges and margin." 



