the Palaozoic Bivalved Entomostraca. 415 



enter on a distinct genus, judging from the general shape and 

 character of some of these minute valves associated with others 

 that are furrowed and, from similarity of style and structure, 

 apparently congeneric with them. We allude to the dwarf 

 forms of Leperditia, on one hand, where relative size is often the 

 only available basis of distinction (for the eye-spot may be want- 

 ing, the muscle-spot invisible, and the flanged edges may be 

 hidden), and, on the other hand, to those true Beyrichue with a 

 single sulcus, and in which the second sulcus is reduced to a 

 minimum or altogether wanting (B. arcuata, &c). In this case 

 also relative size is distinctive, as well as, perhaps, the difference 

 of geological horizon. 



Another difficulty is found in defining the probable alliances 

 of some minute Silurian Entomostraca which have been grouped 

 under " Cytheropsis," and which in outline agree with Primitia, 

 but want the sulcus altogether. It is possible, however, that 

 the sulcus is not an essential character zoologically, and that the 

 merely slight impression (as in P. Beyrichiana and P. obsoleta) 

 leads us to altogether non-sulcate forms : and here the balance of 

 probabilities, judged of by the general aspect of the specimens, 

 must be our guide ; and we must still be content with imperfect 

 classification, if we wish to make our present knowledge of these 

 little Pakeozoic fossils available. If, therefore, they are to be 

 catalogued and brought into relationship with their larger con- 

 temporary allies and their modern representatives, we must ac- 

 cept and make the most of such features as are apparent, and 

 give credit for probable divergency in the unpreserved soft parts 

 when the valves show differences of contour, foldings, and sculp- 

 ture. Hence we have been induced to value more highly than 

 formerly the differences in recorded varieties of the so-called 

 Beyrichia strangidata (Annals Nat. Hist. Sept. 1855), and we 

 shall offer diagnoses for them accordingly. 



Primitia, gen. now 



Carapace minute, bivalved, either equivalved or nearly so, 

 convex, more or less oblong, often approaching Leperditia in 

 shape, by the sloping of the dorsal angles ; hinge-line straight, 

 sometimes nearly as long as the valve. Surface of each valve 

 usually impressed on the dorsal region, either medially or to- 

 wards the anterior extremity, with a vertical sulcus, variable in 

 size, sometimes barely visible, sometimes passing into, or even 

 merely represented by, a navel-like pit ; and sometimes the 

 sides of the sulcus are swollen, and even raised up into tubercles*. 



* With some additional details, the diagnosis for " Beyrichia: simpliccs," 

 given in Ann. Nat. Hist. ser. 2. vol. xvi. p. 85, serves for Primitia. 



