164 SYNOPSIS OF THE CHARACTERS OF THE 



DiTHYROCARIS TENUISTRIATUS. M'Coy. (PL XXIII. fig. 3). 



Sp . Ch . — Valves elongate, ovate, rounded anteally, obtusely pointed retrally, convex ; mesial ridge large, run- 

 ning nearly the entire length of the valve ; two smaller ridges close to, and parallel with the inner margin ; about 

 one-fifth of the length from the anterior end, and situate between the mesial and internal ridges, there is a 

 short sicrmoidal ridge, and a fifth one at the anterior end defines, for a short way, the line which separates the 

 flat, external margin from the convex part of the valve ; surface finely and regularly striated longitudinally. 



This species differs from all others of the genus, in the want of the retral spine to the valve ; it is also much 

 more convex than any of its congeners ; fragments may be discriminated by the regularity and fineness of the 

 striaj. Length of the valve one inch, width five lines. 



Entomoconchus. M' Coy. 

 Gen. CIi. — -Shell bivalve, globose ; depth from the dorsal to the ventral margin exceeding the length ; abdo- 

 minal margin most convex ; hinge auriculated at each end, and with two small, beak-like folds nearly in the centre. 



Entomoconchus Scouleri. 31' Coy. (PI. XXIII. fig. 4). 



Entomoconclius Scouleri. M'Coy, Jour. Geol. Sec. Dub. — Cytherina Phillipsiana. Koninck. Crus. Bel. 



Sp. Ch. — Globose ; hinge in the direction of the shortest diameter of the shell, having at each end a short, 

 square, or slightly rounded ear ; one end more globose than the other, the straightest extremity having a slightly 

 developed auriform process, resembling those of the hinge ; surface very finely granular. 



A descrijjtion of this fossil has already been published in the Transactions of the Geological Society of 

 Dublin ; it was originally figured by Prof Phillips in the Geol. York., but not named or described ; subsequently 

 to the period at which I described it, Koninck figured and described it under the name of Ci/therina Phil- 

 lipsiana. I have retained my own name, not so much from its priority, as because I do not think it can be 

 well referred to Cytherina; each valve, besides the small, square ears, has two obtuse umbones close to the 

 hinge-line ; the general form reminds us of shells of the genus Lima, as the recently discovered genus, Isaura, 

 puts on in the same family the appearance of a Cyclas. Those shell-like Entomostraca are of the greatest 

 interest, many of the minute bivalves suffered to remain in genera of Conch if era belonging almost certainly to 

 this class. Length eleven lines, width nine lines. 



Daphnia PRiM^VA. MCoy. (PI. XXIII. fig. 5). 



Daphnoides (?) Hib. Trans. Roy. See. Ed. 



Sp. Ch. — Longitudinally oval, compressed; without tail; head forming a short, obtusely pointed beak, 

 curved downwards. 



I believe this is the first instance in which the genus Daphnia has been observed in the fossil state ; the 

 present species is not very uncommon in some localities; like one or two of the recent species, it is without the 

 posterior mucronate point, possessed by the majority of the genus; the head forms a very distinct, but short 

 beak ; but I have as yet seen no traces of its eye, although the shell on that portion of the surface is well pre- 

 served; the sides are but slightly convex. Length one and one-third lines, depth three-fourths of a line. 



Bairdia. ilf ' Coy. 



Gen. Ch. — Shell elongate, fusiform, suddenly tapering at both ends; a very short proportion of the valve 

 overlaps the abdominal margin. 



The fossils of this genus are readily distinguished by their lengthened, ovate form, and ends suddenly taper- 

 ing to blunt, recurved points ; they arc all of a small size. I have named the genus in honour of Mr. Baird, 

 the able investigator of the living British Entomostraca. 



Bairdia curtus. M'Coy. (PI. XXIII. fig. G). 

 Sp. Ch. — Length twice and a half the depth ; dorsal margin very convex; abdominal margin nearly straight. 

 This is a much shorter and more gibbous species than the next. Length one line. 



