344 Dr. H. Woodward — Cirn'pedi's/ro»i Trimmuujham Chalk. 



(marked with lines of growth) here forms a slightly prominent 

 ridge." The inner basal angle of the specimens now before me is 

 rather more produced to an acute angle than in Darwin's figure. 



Latera. — The upper pair of latera (Figs. 21, 22) are much longer 

 than those of the series forming the lower whorls ; ' they are flat, 

 very narrow, tapering, and acutely triangular in form, the summits 

 being worn smooth ; the rest of the exterior surface is oruamented 

 by about twenty raised transverse ridges or periodic lines of growth, 

 giving them a somewhat similar appearance to the ornamentation on 

 the surface of the scutum and tergum. These lines of growth pass 

 over the edges of the lateral borders, and are continued on the inner 

 surface of the valve, where they curve upwards, meeting iu a raised 

 median longitudinal ridge, which is frequently prolonged to the apex 

 of the latera. Width of larger latera 3 mm., length 13 mm. 



Figs. 19-24. — Brachylepas (PoUicipes) fallax, Darwin, sp. 

 Trimminshain, Norfolk. 



From the Chalk of 



Figs. 19, 20. Carinal latera, or sub-carinoe. x 2 nat. size. 

 Fig. 21. Latera, external view, x 2 uat. size. 

 Fig. 22. Latera, internal view, x 2 nat. size. 

 Figs. 23, 24. Sub-latera. x 3 nat. size. 



Basal latera. — Two basal latera found associated with the above 

 measure 3 mm. in length by 2 mm. in breadth ; they are marked by 

 arching strife. (See Figs. 23, 24.) 



Dimensions of separate valves of B. fallax from Trimmingham : — 

 Scuta. — There are eight examples of scuta. I give measurements 

 of six : — 



Colin. 



1. R. M. Brydone 



2. R. M. B. ... 



3. R. M. B. ... 



4. R. M. B. ... 

 .5. R. M. B. ... 

 6. Geol. Surv. Mus. 



Length in mm. 



. . . 18 (apex to angle of lowest curve) 



... 17 (pair valve) 



... 10 (small specimen) 

 ... 8 (small specimen) 

 ... 8 (small specimen) 

 . . . 8 (specimen imperfect) . . . 



Breadth in mm. 



11 (greatest). 

 (injured on lat. marg.). 

 6 

 5 



5| 

 5 



* In his description of Mitella fallax from the Maestricht Chalk, Mens. Bosquet 

 says these latera are three times as long as the largest of the lower whorls and many 

 times larger than those of the lowest and smallest series, op. cit., p. 20. On his 

 pi. ii, figs. 7«, b (op. cit.), Bosquet figures a latm precisely similar to Mr. Brydone's 

 specimen from Trimmingham, Figs. 21, 22, sitpra. 



