24 THE ENTOMOSTRACA OF 



anterior and posterior thirds, forming a slight projection^ at each extremity of the thin, 

 straight hinge-margin ; ventral edge inverted along its central third. 



Dorsal aspect acute, oval ; anterior oval. 



In the recent specimens there is in the middle region of each valve an opacity- 

 occupying an irregular space, in the lower part of which, and below the centre of the 

 valve, is situated a transparent rosette, formed by six or seven three-sided lucid spots, 

 placed around a central . round spot. These spots are externally pits, and internally 

 faint eminences. Many of the fossil valves retain traces of the opacity and the 

 lucid spots. 



B. subdeltoidea differs from B. curta and B. gracilis, M'Coy,^ judging from the 

 figures and descriptions, by being, especially as regards the latter species, less acute 

 anteriorly, much wider between the dorsal and ventral borders, and by its valves 

 being much more gibbous. 



M. Bosquet has referred to some points in which C. trigona. Bosquet, differs from 

 M. Roemer's figures of C. subdeltoidea f" but we cannot think that there is any essential 

 dissimilarity between the two. M. Bosquet's figure 2>\b, intended for the right valve, 

 is evidently the left valve in a reversed position. The right valve is strikingly distinct 

 from its fellow valve, and from the figure alluded to. 



This species is described by Miinster as occurring in Tertiary strata at Osnabruck, 

 Paris, Bordeaux, and Castellarquato, in the Middle strata of the Chalk Formation, 

 near Dresden, Miinster, and Lemforde, and in the Chalk at Maestricht. M. Rcemer 

 found it in the Lower Chalk ; Dr. Reuss found it common throughout the Chalk 

 Formation of Bohemia, especially in the Lower Exogyra Sandstone, and particularly 

 abundant in the " Planer-schichten." According to M. Bosquet, it is rather common 

 at Maestricht and the neighbourhood. Though abundant in the Chalk and Chalk- 

 marl, and not rare in the Greensand, this species does not occur in the Gault. This 

 is a common form in the Tertiary deposits, and it is very plentiful in the tropical 

 seas ; a narrow variety of this species exists in the Firth of Clyde. 



We have not met with this species in any of the Oolites, but closely-allied form.s 

 occur in the Magnesian* and Carboniferous^ Limestones of the British Isles, and 

 in the Bituminous Limestone (Carboniferous) of South Australia.® 



1 Fig. 15/'/"/'"; a the anterior, and b the posterior projection. 



2 Syn. charact. pp. 64, 65, pi. xxiii, figs. 6, 7. 



3 In the Jahrbuch, 1838, pi. vi, fig. 16, the right valve is figured, and the left valve in the Verst. 

 Nordd. Kreid. pi. xvi, fig. 22. 



* In the collection of Mr. King, Newcastle. 



5 In greyish Limestone, at Lawrieston and Stuartfield (East Kilbride), also in Limestone, near Newcastle- 

 on-Tyne. History of Rutherglen and East Kilbride, by David Ure, A.M., 8vo, Glasgow, 1793, pp, 311, 

 312, pi. xiv, fig. 20. Bairdia curta and B. gracilis, M'Coy, before referred to, belong to the Carboniferous 

 System of Ireland. 



6 Physical Description of New South Wales, by P. E. de Strzelecki, 8vo, London, 1845, p. 291, 

 pi. xviii, fig. 10. Bairdia affinis, Morris. 



