4 H. Woodivard — On Prosopon mammillatum^ 



Plagiophthalmus, Bell, would thus probably include within it the 

 following species of H. von Meyer's genus Prosopon, namely : — 



P. hebes, P. simplex, P. rostratum, P. spinosum, P. elongatum, P. de- 

 pressum^ P. ohtusum, P. Stotzingense, P. tuberosum, P. sublaeve, P. laeve, 

 P. punctatum. The following are doubtful : P. insigne, P. cequilatum, 

 P. marginatum, P. grande, P. excisum, P. lingulatum.. 



For the remainder, the generic name Prosopon should be retained, 

 namely : P. aculeatum, P. ornatum, P. paradoxwn, P. torosum, P. Meg- 

 dent, and P. (Bquum. 



Having only seen actual specimens of a few of these forms I do 

 not wish, at present, to do more than indicate those species, which, 

 I think, will need revision. They all occur merely as detached cara- 

 paces, without appendages, and the under-side of the fossil usually 

 adheres firmly to the matrix and is therefore seldom seen. 



The crab now to be described (Plate I. Fig. 2), although larger 

 than the Wurtemburg specimens, is no doubt referable to the genus 

 Prosopon in its restricted sense. 



It was first noticed by Professor Morris, F.G.S., who obtained an 

 imperfect carapace many years since ; it was next observed by Mr. 

 Samuel Stutterd, of Banbury," who found a portion of another spe- 

 cimen, which he kindly brought to me for examination. Lastly, for 

 the very perfect carapace, now figured, I am indebted to George 

 Griffith, Esq., M.A., the Assistant- General Secretary of the British 

 Association, who procured it at Stonesfield, from whence the two 

 other examples, above referred to, were also obtained. 



The specimen measures fourteen lines in length, and eleven and a half lines in breath. 



The front of the carapace is four lines in breadth, and is marked by a semi-circular 

 depression in the centre, and by two laterally diverging horns— similar to those which 

 ornament the front of the carapace in many of the Triangular crabs. ^ Immediately 

 behind these horns are placed the orbits which are bounded on their exterior angles 

 by short blunt spines. Here the carapace measures six lines across ; the hepatic 

 region then swells out into a tumid prominence, ornamented by a single spine, the 

 breadth of the carapace being increased to nine and a half lines. Posterior to the 

 hepatic region the cervical or nuchal furrow crosses the carapace, forming a deep in- 

 dentation between the gastric and cardiac regions. A second transverse furrow, three 

 lines behind the cervical furrow, indents the carapace and unites with the cardiac 

 furrow on either side. 



The regions of the carapace are all well-marked and very tumid : the gastric region 

 is the most prominent, and is marked by two mammillae (which have suggested the 

 trivial name). When viewed in profile (see PL I. Fig. 2 a) these mammillae are 

 elevated three and a half lines above the level of the slab on which the carapace 

 rests. The posterior border of the carapace is nine lines in breadth, and but slightly 

 curved. The branchial and cardiac regions of the carapace are covered with minute 

 rounded tubercles. The gastric and hepatic regions are also tuberculated ; but the 

 tubercles are fewer and of larger size. 



In a paper, communicated to the Geological Society, May 23rd, 

 1866 (which was published in the Quarterly Journal, vol. xxii. 

 p. 493, pi. xxiv. fig. 1), "On the oldest known British crab (^Palce- 

 tnachus longipes) from the Forest Marble of Malmesbury, Wiltshire," 

 I pointed out the synthetic characters which these early forms of 

 Crustacea present. In Prosopon mammillatum we have another 

 example of this blending of characters in the outset of the Bra- 

 chyura in Oolitic times. 



^ The Macropodiada of Milne-Edwards. 



