Mr. C. W. Peach on the Luminosity of the Sea, ^c. 425 



nular surface. The figure agrees with the shell figured by Phillips ; 

 the greater number of specimens do not exhibit any markings upon 

 the surface. 



233. CucuUcea nana ; minute, rather compressed, suborbicular ; 

 umbones mesial, touching ; hinge-line short, rounded at the extremi- 

 ties ; surface with extremely fine decussating lines, two or three of 

 which upon the posterior side are more elevated. 



249. CucuUcea bipartita ; small, rhomboidal ; umbones large, me- 

 sial ; hinge-line angvdated at its extremities ; a longitudinal oblique 

 keel upon the posterior side, and a wide and deep depression extend- 

 ing from the umbo to the ventral border ; surface with lines longitu- 

 dinal, closely arranged, crossed by a few lines of growth. 



234. Lithodomus attenuatus ; elongated, smooth, posteriorly at- 

 tenuated ; umbones small, near to the anterior extremity ; width |ths 

 of an inch, which is thrice the length. 



248*. CucuUcea obUqua ; depressed, oblique, subtrigonal ; um- 

 bones touching, small, mesial ; hinge very oblique to the ventral 

 border ; anterior side short, rounded ; posterior produced, flattened 

 and angulated at the base ; surface with extremely fine lines, both 

 longitudinal and transverse, the latter very densely arranged. There 

 may sometiiues be difficulty in distinguishing this from CucuUcea cu- 

 cuUata, but the present shell is more flattened and oblique, the um- 

 bones are much smaller, the hinge-line shorter, and the posterior 

 border more lengthened. 



1 72. Trigonia clavo-costata ; shell elevated, anterior border round- 

 ed ; costse regular, some tuberculated, others smooth ; tubercles large, 

 closely arranged, the first three or four and the last one or two ele- 

 vated, but without tubercles ; cardinal area broad, flattened, with 

 oblique carinte ; posterior extremity short and truncated. This shell 

 has usually been confounded with T. claveUata, but the figure is 

 much more truncated or shortened posteriorly, the costse are perfectly 

 regular, and the tubercles are very large and closely arranged, the few 

 first and last costae being simple ; these several features separate it 

 from the Oxford clay species. It approaches near to T. Brotinii, 

 Agassiz, in form, but the character of the costse as above described 

 are different. 



XXXV. — Observations on the Luminosity of the Sea, with desanp- 

 tions of several of the objec'.^ vw,<.ich cause it, some new to the 

 British Coast. By Charles William Pbach, Associate of 

 the Royal Institution of Cornwall*. 



[With three Plates.] 



The Report of the Royal Institution of Cornwall for 1846 con- 

 tained some remarks of mine on the luminosity of the sea; since 

 that time I have taken every opportunity of extending my ob- 



* Read November 2nd, 1849, and abridged from the Report of the In- 

 stitution. 



