1 8 AN INTRODUCTION TO THE STUDY OF FOSSILS 



Utica shale in New York State furnished not only the legs, anten- 

 nae and gills, but traces of the digestive tube as well (Fig. 126). 

 The Lithographic limestone in Bavaria and the Oxford clays in 

 England show by the preservation of arms, outline of body and 



A B 



Fig. 4. — The intimate relationship which exists between the hard shell and the 

 soft body of an animal is seen here in the scallop, Pectcn gibhus borcalis, from the 

 New England coast. A, outer surface of left or upper valve ; B, right valve, with 

 the left showing slightly at the base. The mantle secreting the right valve (B) 

 received an injury when the edge of the shell was the growth line a-a' ; this injury 

 lowered at once the vitality of the entire body, so that anterior growth of both 

 mantles wholly ceased for a time, a fact recorded in the conspicuous growth hne 

 a-a' . Though the animal lived for some time after this injury it never regained 

 its former vitality, as is shown in the development of such old age characters as 

 the obsolescence of the ribs and the irregularity of the growth lines. The mantle 

 at {a) upon the right valve ceased growing entirely but the opening thus made 

 was partially filled by the inbending of the opposite valve. (Slightly reduced.) 



ink sac that the Mesozoic belemnoids were very similar to the 

 living squid, while in the shale of the Stephen formation of 

 Alberta, occur beautiful impressions of such soft-bodied animals 

 as jelly-fish, worms and sea cucumbers. In trilobites there is 

 usually preserved only the hard dorsal shield ; in belemnoids 

 only the small, internal skeleton ; in worms, teeth ; and in sea 

 cucumbers scattered calcareous plates. 



In all vertebrate animals muscles are fastened to an internal 

 skeleton by projections, roughened surfaces or depressions upon 

 the latter; the larger such surface or projection, the larger the 

 muscle attached to it. Hence from the bone alone the number 

 and size of the muscles formerly attached to it can be estimated. 



