﻿BYSSIFEILE. 



181 



perished from the attack of some predaceous species. As this does not coincide with 

 the usual condition of fossil tests of this genus, a special cause is required to account 

 for it. The byssal aperture indicating a fixed or sedentary condition offers an explanation ; 

 the adult mollusks fixed by their byssal appendages had not the free saltatory motions 

 of other Trigonia, and became victims to the marine flesh-eaters which attacked the 

 siphonal or thinner and more exposed upper portions of the valves. The appearance of 

 the lines of growth indicate that the byssal orifice was formed at the completion of 

 adult growth ; the shell generally at that period, and more especially the area, exhibits 

 commonly an abraded aspect similar to the appearance of adult Bysso-Arcas, and 

 probably produced by similar conditions of existence. Usually, therefore, it is only upon 

 immature specimens that the surface ornaments of the area have been fully preserved, 

 and even in such instances the posteal portion had no longer distinct carinae or costellae, 

 their positions are occupied by transverse rugose folds. 



Some alteration in the figure of the area and escutcheon took place during the 

 progress of growth ; the young specimen, figure 4 a, has the escutcheon comparatively 

 small and concave ; the surface of the area equals the other portion of the valve, so that 

 the length of the siphonal border is equal to the hinge-border; in the adult slate the 

 escutcheon extends fully three fifths of the posteal slope, and the siphonal border is com- 

 paratively short. 



The figures of T. carinata, given in the work of Agassiz (' Trigonies,' tab. vii, figs. 

 7—10) represent two small specimens deprived of their tests ; the surfaces of the area and 

 escutcheon are not preserved, and the costse are only faintly indicated. T. sulcata of the 

 same work (tab. viii, figs. 5 — 7) represents an internal mould of T. carinata. Fig. 8 is 

 a large example deprived of the test ; it has traces of the oblique costae, but has no por- 

 tion of the surfaces of the area and escutcheon ; figure 10 represents a very young 

 specimen destitute of any portion of the surface ; the species, therefore, is very inade- 

 quately illustrated by the figures of these two supposed species. 



The figures of T. carinata given by d'Orbigny (' Paleont. Fran. Terr. Cret.,' plate 286), 

 although affording beautiful examples of lithographic art, are not altogether satisfactory 

 representations of the species. The larger of the specimens is not of adult growth, but 

 should represent the changes which had taken place over the posteal portion of the area 

 in the effacement of the carinae and intercarinal costellae by rugose irregular plications ; 

 on the contrary, the carinas, including a delicate median carina and the intercarinal 

 costella?, continue prominent even to the siphonal border ; there are no differences in the 

 intercarinal spaces ; the escutcheon assumes the aspect of a deep concavity, and the 

 costae although prominent are without the posteal terminal attenuations. These several 

 features differ essentially from the more perfectly preserved examples of the species from 

 Atherfield, so celebrated for the fine condition of its Neocomian Testacea.' The descrip- 

 tion in the text of the same work is so concise that it does nothing to rectify the 

 incorrect details of these figures. 



24 



