1858.] MUECHisoisr — noethekn highlands, etc. 405 



direct analogies to the bivalved carapaces of certaia recent Phyllopodous Crus- 

 taceans inhabiting the rivers and lagoons^ of hot countries, and often much 

 resembling the shells of Nucula, Cyrena, &c. 



G-reat numbers of the valves are spread over large surfaces of the flagstone, 

 sometimes scattered sparsely, sometimes congregated in groups, forming jfilms 

 between the layers of the fissile stone. Sir Eoderick remarks (above, p. 404) 

 that their distribution appears to be very local ; — I have not learnt how thick the 

 band, or bands, of the flagstone may be that they affect. The valves are usually 

 single ; pairs, with their hinge-lines in juxtaposition, are rare. The specimens 

 which I have are in dark-grey, tough, fine-grained, sandy flagstone, slightly mi- 

 caceous, somewhat varying in tint and hardness. They usually appear to have 

 a superficial smoothness or even gloss, and often a light-brown tint, with some 

 degree of translucency. But the substance of the valve flakes off readily, leaving 

 a film on each of the two surface-planes in a spht stone ; and it is comparatively 

 seldom that a valve shows its real exterior ; for, though the surface may some- 

 times come away from the stone in splitting, and leave a clean cast, yet an outer 

 flake of the valve seems nearly always to have fallen away with the shock of the 

 blow ; and the sandy natiu'e of the matrix is too coarse to retain traces of any 

 very fine microscopic scidpturing in the cast or mould of the surface. 



In the Kirkwall specimen the valves are pyritized ; but those from Caithness 

 retain their brown horny tissue, although the outermost surface is seldom pre- 

 served, and only in little patches on some valves. From this circumstance it 

 is very difficult to form a correct diagnosis of the species ; for the superficial 

 ornamentation yields important specific characters in this genus, and in this 

 case it cannot be well studied. 



A careful examination of the materials at my command enables me to describe 

 the carapace as follows : — 



Valves variable in shape from a subquadrate to an oblong form, — some being 

 about ^th inch long and ith inch high, and others being as high as long. The 

 hinge-line is straight ; the generality of the valves have the anterior and posterior 

 edges forming sharp angles with the dorsal line and passing vertically with a 

 shghtly convex outline to the boldly-rounded ventral border. In this case the 

 umbo is distinct, almost in the middle of the back line, and bordered by a tri- 

 angular depressed ear on either side; something like a miniatiu'e Cucullaa 

 auriculifera. 



There are also valves which are longer and narrower, with the ends rather 

 more convex, and the ventral border somewhat straight. Here the umbo is less 

 protuberant and nearer to one end (the anterior?). 



In each of these forms the surface is wrinkled, by about 18 to 20 concentric, 

 rounded, closely-set ridges, uniform with the outline of the ventral border and 

 extremities of the valve. The ridges are coarsest near the imibo, their starting 

 point, and they become finer as they approach the edges of the valve. 



Under the microscope, the patches of the outer surface, here and there retained, 

 are seen to be of a dark-brown opake substance, exhibiting on and between the 

 ridges a finely granular appearance, which is probably the real condition of the 

 surface ; or it may be due to an accidental modification of an originally minute 

 reticulate ornament. 



As it is possible that the difference in shape between the many subquadrate 

 and the few oblong valves may be due to pressure, to a condition of growth, 

 or perhaps to difference of sex, it would be too hazardous even to distinguish 

 them by name as varietal forms f. Indeed, circumstances have so modified the 

 great majority of the valves, that the whole outline of a valve can very rarely 

 be definitely traced even among a hundred individuals; for the edges either 



* The Esther ia minuta of the Trias is the subject of a note in the Quart. 

 Journ. Geol. Soc. vol. xii, p. 376 ; and I have there referred to the Caithness 

 form. The information, accompanying a recent specimen, which led me to term 

 Estheria a " marine" crustacean {op. cit. p. 377), is, I find, probably erroneous, 

 or at least requires to be substantiated. 



f In one or two instances I was almost misled by apparently elongate valves, 

 which were, in truth, two valves pressed accidentally one on another " con- 

 formably," but still one extending a little beyond the other. 



