Abbreviations. 



.1/. ert., the extenial wiiig in tlic ('l_vi)earitriti:p. 



^■1/. htf., tlie inniT wing of the latm^al iiiter-iiiuscular sheel, separated hy the 



triaiigle, fr., from the al. c.xt. 

 Aur.. an auricle or its brani'h. 

 Caii.. tlie eeutral canal in tlie alveole. 

 Cond., the condyloid eniinence of tlie rotula. 



Corp., curpus, the eonipact inidiUe body of the pyramid in llie Clypeastridse. 

 Cr., the supra-alvc(dar erest. 



D., the tooth. 



Ep., the epiphysis. 

 Fo., the fossa of the jjyramid in Clypea.ster. The fossa of the rotula in 



the Regnlaria. 

 CtJ.. the glenoidal eavity of the epijihysis. 



Int., the intestine. 



Lam. int.. the lateral sheet of lin' i)yrami(l, to wliieli the iuter-pyramidal 



muscles are attaehed. 

 3/. inf; the inter-jiyraniidal muscles. 

 M.phar., the external muscles of the i)harynx. 

 M. pro., the protractor muscles of tlie pyramid. 

 M. re. the retractor muscles of the pyramid. 

 M. ro. c. aud )». ro. i. the external and internal rotular muscles, tig. 42, ii, 



45, 4i;. 

 Os., the mouth. 



Pe. hur., the Imccal pedicels. 

 Pr. lab.. the labial process. 

 Pr, sup., the supra-alveolar jirocess. 

 Rh., the rachis. 



Po., the rotula. 



Sept., the septum between the fossa» in Clypeaster. 

 Sph., a spherid, tig. 80, hfi. 

 S. de., sulcus dentalis, the dental slide. 

 Su. med., the mesial suture of the pyramid. 

 Syniph.. the symphysis of the half-pyramids. 



Tr.. the triangulär area that separates the wings in the Clypeastridae. 



Tu. i., and tu. e., the tubercles, internal and external of the epiphysis, fig. 



42, 49, 51. 



