204 AN INTRODUCTION TO THE STUDY OF FOSSILS 



sinus. Brachial valve with very low cardinal area and strong 



median fold. Surface of entire shell radially plicate or striate, 



or with the fold and sinus not plicate, crossed by more or less 



^,^ strong growth lines. 



^^m-'—^^^>y' are directed towards the 



liG. 86, — Spirifer mucronatus Conrad, from the cardinal ano^les (hence 



Hamilton (mid-Devonian) of New York, show- , /^fV. f fVi 



ing the long, tapering spirals attached to the ^^^ great Wldtn 01 tne 



inside of the brachial valve for the support of shell) ; SOme Spirifers 



the brachia. Natural size. (After Hall and , • i r 



Clarke.) have spirals of 35 rev- 



olutions. The animal 

 was anchored by a short pedicle w^hich passed out through 

 the delthyrium. The genus Spirifer of authors has been sub- 

 divided into many genera made necessary by the variations 

 in the vast number of species found. 



S. mucronatus is a characteristic Middle to Upper Devonian 

 species of North America ; 5. disjunctus, an Upper Devonian 

 species throughout North America and most of the world; S. 

 cameratus, a widely distributed North American Pennsylvanian 

 species. 



1. Sketch (a) side view of entire shell, (b) exterior of pedicle 

 valve, (c) view looking directly upon the hinge line, (d) interior 

 of brachial valve showing the spirals. Indicate in sketches the 

 valves, plications, growth lines, median sinus, median fold, 

 hinge line, cardinal areas, delthyrium, spirals. 



2. What is the relation of the mantle to the shell ? 



3. What was the function of the spirals ? 



4. How do you distinguish this genus externally from Platy- 

 strophia ? 



5. What is the significance of the name Spirifer ? 



6. What is the geologic range of the class Brachiopoda ? 



7. During what periods did the greatest number of brachiopods 

 live? 



8. How many species are living at present ? 



