﻿COSTATiE. 17a 



Tbigonia Culleni, Lye, sp. nov. Plate XXXI, figs. 9, 9 a. 



The Marine intercalated Millepore bed, a subordinate member of the Lower Sand- 

 stones and Shales at Cloughton, a few miles to the northward of Scarborough, has 

 associated with Trigonia recticosta and other testacea of the Inferior Oolite, a small 

 costated species of Trigonia, and as the matrix is usually very hard and the tests of the 

 Trigonia are either delicate or indifferently preserved, it rarely happens that a specimen 

 can be separated sufficiently entire to allow of the clear development of its characteristic 

 features ; thus it happens that our sole illustrative specimen is imperfect ; it appears, 

 however, possible by its aid to describe the species and separate it from other allied 

 forms of the C'ostatce. 



The most prominent features consist in the form unusually lengthened transversely 

 and posteally ; in the considerable recurvature of the umbones together with the produced 

 and rounded anterior side ; also in the minuteness of the lengthened horizontal rows 

 of costse, the specimen figured having upwards of twenty-seven ; they are elevated, and 

 so narrow as to appear almost hnear, viewed from the anterior side, and with the valves 

 united. The horizontal rows are, however, distinctly separated, and are almost without 

 any anteal undulation. 



The upper border of the shell is concave ; it forms an obtuse angle with the siphonal 

 border, which is oblique, and has considerable length ; the lower border is lengthened and 

 is only slightly curved. 



The escutcheon and area are large and have no clear separation, the former is 

 deeply excavated ; the inner half of the area is also concave ; the costellse upon their 

 surfaces are minute and very irregular. The marginal carina is small, but is distinctly 

 transversely plicated. 



The specimen figured has the length 15 lines; the height 10 lines; the diameter 

 through the united valves 9 lines. 



Named after its discoverer, Mr. Peter Cullen, a veteran and intelligent collector of 

 fossils at Scarborough, the greater portion of whose life has been occupied in the 

 development of the palgeontological treasures of the district in which he resides. 



Affinities and differences. T. hemisphcerica with similar minute costse has the general 

 figure much shorter and less produced anteally, the umbones are much more nearly 

 terminal and have little recurvature ; the surface ornaments of the area and of its carinse 

 are much larger and more deeply sculptured. Another small gregarious example of the 

 CosfatcB also occurs at Cloughton ; the test has usually decomposed, but the ribbing is 

 distinct ; its fewer and larger costs separate it from the present species ; it may represent 

 the very young condition, or a variety of T. denticulata. 



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