156 AN INTRODUCTION TO THE STUDY OF FOSSILS 



thus uniting across the straight suture of two contiguous plates 

 form in outUne a rhomb, from which these have received the 

 name of pore-rhombs. The function of the pore-rhombs is 

 supposed to have been respiratory. 



Caryocrinus is thus somewhat similar to a starfish turned 

 mouth side uppermost, with an anchoring stalk attached to the 

 dorsal side, the mouth and much of the ambulacra concealed by 

 calcareous plates, and with the ends of the ambulacra branching 

 into free arms. With the mouth concealed, food was of neces- 

 sity urged into it through the ambulacral grooves by the cilia 

 lining them. Since the ambulacra were often concealed by 

 calcareous plates, tube-feet were in all probability absent. 



1. Sketch (a) side view, (b) enlarge a plate twice. Label 

 calyx, place for attachment of stem and of arms, plates forming 

 the ventral surface of calyx, pores. 



2. What was probably the function of the pores ? 



3. Of what did the animal's f^od probably consist ? How did 

 it procure it ? 



4. Compare Caryocrinus to a 

 starfish. 



5. What was its habitat? 

 Reasons. 



6. What part of the living 

 animal does the fossil represent ? 



7. What animals living off our 

 coast are likewise fastened to 

 the ground ? 



8. What characters show the 

 cystoids to be primitive un- 

 specialized types ? 



9. Give the geologic range of 

 the cystoids. 



amb. 



an. 



Fig. 60. — A cystoid, Agelacrinus cin- 

 cinnatiensis Rcemer, from the Ordo- 

 vician of Cincinnati, Ohio. Top view 

 ( X 2) showing the curved ambulacra 

 {amb.) within which are the covered 

 food grooves. Anus {an.) nearly mar- 

 ginal. (Redrawn from Meek.) 



Agelacrinus (Fig. 60). 



Ordovician to Mississippian. 

 The animal is circular, flat, 

 stemless, cemented by the entire 

 lower (dorsal) surface to a foreign object, usually a brachiopod 



