2 FOSSIL MALACOSTRACOUS CRUSTACEA. 



Obs. The above description is taken from two specimens, existing respectively in the 

 British Museum and in Dr. Bowerbank's collection, in each of which the carapace alone 

 remains. The difficulty which necessarily exists in obtaining good generic characters 

 from such slight data is but too well known and too often felt by every palaeontologist ; 

 and it is especially the case in the class of Crustacea, where the generic characters are 

 ordinarily derived from parts which are most easily destroyed, and Avhich, even when 

 existing in a perfect condition, require the most acute discrimination to detect their 

 essential structure and determine their physiological bearing and importance. 



This species was first described by my friend, Mr. Charles Gould, in the 'Quarterly Journal 

 of the Geological Society,' published in May, 1859, where he has the following observa- 

 tions : — " I regard the Crustacea, which I am about to describe, with peculiar interest, on 

 account of its belonging to the group of the Brachyura, which comprehends the highest 

 forms of the class, and of which hitherto but one species {Mithracia Ubinioides, Bell*) has 

 been described as occurring in the fossil state in Great Britain.'' 



All the specimens hitherto known are from the lower Greensand in the Isle of 

 Wight, 



Genus — Trachynotus, Bell. 



Species unica. Trachynotus sulcatus. Plate I, fig. 1. 



Descr. Carapace convex, subrhomboidal, with rounded angles, rather broader than 

 long, the transverse diameter widest immediately behind the mesobranchial lobes; the 

 portion anterior to the nuchal furrow occupying scarcely one third of the entire area ; 

 the surface is covered with small tubercles, which, for the most part, are irregularly 

 arranged in rows upon transverse ridges, which are separated by strongly marked furrows ; 

 the nuchal furrow in its course across the carapace bends a little backwards, forming a 

 very obtuse angle ; a second furrow, parallel with it, extends entirely across the carapace, 

 and a third, crossing the mesobranchial lobe, terminates by passing forwards into the one 

 anterior to it, at some distance from the median line. The metagastric lobe forms a 

 regular rhomb, its anterior process is linear and extends to the front ; the latero-anterior 

 margin is rounded and without teeth or other armature, the latero-posterior margin is 

 hollowed, and the posterior has a narrow, slightly raised, border. 



Length of carapace, 0-5 inch; breadth, 06 inch. 



From the upper Greensand of Wiltshire. 



Obs. Of this singular species I have seen but two specimens, one in the British 



* 'Monogr. Foss. Malac Crust. Gr. Brit.,' Part I, "Crustacea of the London Clay," 18;">8, p. 9, pl.v, 

 figs. 10—12. 



