from the Fossiliferous Limestone of Durham. 437 



were not well preserved, nor had the characters of the genus been 

 properly defined by its author : but from the data offered by the 

 imperfect materials in his hands, he thought it possible that it 

 might have some afl&nity to it ; so he placed it in it provisionally. 



Dithyrocaris was originally considered by Dr. Scouler to have 

 a univalve carapace, like Apus and other single-valved Branchio- 

 poda*. He afterwards altered his views, supposing it to be 

 bivalve, like Cypris, though differing from that genus in having 

 caudal appendages protruding from the valves. This opinion 

 was held until 1813, when Colonel Portlock described two new 

 species from the shales of the Lower Carboniferous rocks of Ire- 

 landf, and proved that its carapace was univalve, as Dr. Scouler 

 had supposed at first. Colonel Portlock^s description of these 

 species, particularly of D. Colei, can leave no doubt of the cor- 

 rectness of his views in this respect, and clearly demonstrates 

 that Dithyrocaris is a univalve Entomostracan — that is, sup- 

 posing Dr. Scouler's species belong to the same group, which 

 may be taken for granted until proved to the contraiy. 



All the examples of L.t Permiana which came under the no- 

 tice of Mr. Jones were, as stated before, more or less imperfect ; 

 so it may naturally be supposed that great difficulty would 

 occur in attempting to determine the generic affinities of the 

 species to which they belonged. It is evidently owing to this 

 cause that Mr. Jones supposed that it might be a member of 

 Dithyrocaris. It is from the perfect state of preservation of 

 my specimens that I have been enabled to offer the preceding 

 remarks in addition to those of Mr. Jones. And as some of the 

 specimens have the valves united and in close juxtaposition, I 

 have also been enabled to prove that the species was a bivalve 

 Entomostracan, like Cythere, or rather Leperditia, consequently 

 that it has no affinity to Dithyrocaris — nor to Ceratiocaris, to 

 which Mr. Jones afterwards referred it|, its bivalvular character 

 also, of course, excluding it from that genus. 



German and Russian palaeontologists have referred congeneric 

 species to Cythere ; but, with the exception of being bivalve, they 

 possess no characters to warrant their remaining there. 



The true generic affinities of L. ? Permiana and its congeners 

 I leave to be determined by my friend Mr. Jones, as he tells me 

 that he has long been investigating their relations, and as I am 

 quite sure that he is more competent to do so than I am. I am 

 of opinion that they will be found to constitute a new group. 



* Records of Science, Feb. 1835 ; and in a jjaper read before the British 

 Association at Glasgow. See also Jones on Dithyrocaris, in Prof. King's 

 Mon. Perm. Foss. p. 64. 



t Report of the Geology of the County of Londonderry, pp. 313-316. 

 pi. 12. I Morris's Cut. British Fossils, 2nd edit. p. 103. 



Ann. ^ Ma(/. N. Hist. Ser. 3. l^o/. ii. 30 



