406 



FOSSILS OF THE 



[Ch. XXV 



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

 species, such as Spirifer glaber (fig. 523), with its numerous varieties. 



Fig. 522. 



Fig. 523. 



Spirifer trigonalis, Martin, sp. 

 Mountain Limestone : Derbyshire, &c. 



Spirifer glaber, Martin, sp. 

 Mountain Limestone. 



Among the palliobranchiate mollusks, Terebratula 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 are also preserved in several lamel- 

 libranchiate bivalves, as in Aviculopecten (fig. 525), 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. 526), which displays wavy blotches, resembliDg the coloring 

 in many recent Trochidae. 



Fig. 524. 



Fig. 525. 



Fig. 526. 



Terebratula hastata, 

 Sow., with radiating 

 bands of color. 

 Mountain Lime- 

 stone. Derbyshire : 

 Ireland ; Bussia, &c. 



Aviculopecten sublobatus, 

 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 observing, 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. 



