120 Professor J. B. Harrison — Laterite 



second to fifth segments sub-triangular to sub-falcate, with anterior 

 and inferior margins forming a continuous curve and posterior margin 

 dentate. Small triangular area cut off by short furrow from anterior 

 upper corner and swollen into peg for articulation. 



Peraeopods slender, laterally compressed, with smooth surface 

 sparsely dotted with pits. 



Dimensions. mm. 



Length of carapace along median line to rostral notch . . 29'0 

 Length of carapace along median line to level of posterior 



lateral angles ......... 33"5 



Length of thorax along median line ..... 14'0 



Width of thorax posteriorly between lateral carina . . 18'0 

 Width of carapace at point of section of lateral carina by 



cervical groove ......... 13'0 



Distance between outer edges of bases of rostral teeth . . 6'5 



Width of abdomen ........ 14"5 



Holotype. Carapace with five segments of abdomen attached and 

 portions of three peraeopods on right side. From the Lower Greensand, 

 Atherfield, Isle of Wight. 



Affinities. This species differs from Th. Carteri in the more elongated 

 carapace, the smaller development of the lateral keels and their down- 

 ward curvature posteriorly, the much closer and coarser tuberculatum 

 of the surface, the different kind of demarcation of the gastric region, 

 the stronger ridges running back from the bases of the rostral teeth, 

 and in the more convex abdomen. But the "most striking differences 

 amongst those above-mentioned are the general shape and tuberculatum. 



With Th. scyllariformis it agrees more closely than does Th. Carteri 

 in the development of the post-rostral ridges and in the smaller 

 prominence of the lateral carinoe. But otherwise it has fewer points 

 of resemblance. With P. dulmensis the development of the large gastric 

 tubercles is a feature in common, but it is less closely allied to this 

 Senonian species in general characters than is Th. Carteri. 



From the possession of a coarsely tuberculate surface it may be 

 approximately designated by the specific name tuber culatus. 



EXPLANATION OF PLATE VII. 

 Fig. 1. Thenops titberculatus, sp. nov. x %. Viewed from above. 

 ,, la. Ditto. x f . Viewed from right side. 

 ,, lb. Ditto. X#. Viewed from left side. 

 ,, 2. T) h enops Carteri, sp. nov. x f . Viewed from above. 

 ,, 2a. Ditto, x % . Viewed from right side. 

 (Botb specimens are from the Lower Greensand, Atherfield, Isle of Wight.) 



IV. — Ox the Formation of a Laterite from a practically Quartz- 



free Diabase. 

 By Professor J. B. Harrison, C.M.G., M.A., F.G.S., F.I.C. 

 4 FTER I had forwarded the paper on " The Eesidual Earths of 

 xJl British Guiana commonly termed ' Laterite ' ", published on 

 pp. 439-52, 488-95, and 553-62 of the Geological Magazine for 

 October, November, and December, 1910, I visited the neighbourhood 

 of Christianburgh and Akyma on the Demerara River in British Guiana 

 and examined a small long-deserted quarry in a very fine-grained 



