406 



FOSSILS OF THE 



[Ch. XXT 



/S. rotundatus, and S. trigonalis (fig. 522), also abound ; and smooth 

 specieSj such as Spirifer glabsr (fig. 523), with its numerous varieties. 



rigr. 522. 



Fig. 523. 



Spirifer trigonalis, Martin, sp. 

 Mountain Limestone : Derbyshire, «SiC. 



Spirifer glaber, Martin, sp. 

 Mountain Limestone. 



Among the palhobranchiate mollusks, Terebrahda kastata deserves 

 mention, not only for its wide range, but because it often retains the pat- 

 tern of the original colored stripes which ornamented the living shell 

 (See fig. 524.) These colored bands ai'e also preserved in several lamel- 

 libranchiate bivalves, as in Aviculopeden (fig. 526), in which dark stripes 

 alternate with a light ground. In some also of the spiral univalves, the 

 pattern of the original painting is distinctly retained, as in the Pleuroto- 

 maria (fig. 520), which displays wavy blotches, resembling the coloring 

 in many recent Trochid-x. 



Fig. 524. 



Fi?. 525. 



Fig. 526. 



Teretratula hastnta. 

 Sow., with radiating 

 bands of color. 

 Mountain Lime- 

 stone. Derbyshire : 

 Ireland : Eussia, &e. 



Aviculopecten suhlo7>atus, 

 Phill. Mountain Lime- 

 stone. Derbyshire ; 

 Yorkshire. 



Pleurotomaria carinata, Sow, 

 {P.flammigera, Phill.) 

 Mountain Limestone. Derby- 

 shire, &c. 



The mere fact that shells of such high antiquity should have preserved 

 the patterns of their coloring, is striking and unexpected ; but Prof. E. 

 Forbes has deduced from it an important geological conclusion. He 

 infers that the depth of the primeval seas in which the Mountain Lime- 

 stone was formed, did not exceed 50 fathoms. To this opinion he is led 

 by observiDg, that in the existing seas the testacea which have colors 

 and well-defined patterns, rarely inhabit greater depths than 50 fathoms ; 

 and the greater number are found where there is most light in very 

 shallow water, not more than two fathoms deep. There are even exam- 

 ples in the British seas of testacea which are always white or colorless 

 when taken from below 100 fathoms ; and yet individuals of the same 

 species, if taken from shallower zones, are vividly striped or banded. 



