196 ROBERT TRACY JACKSON ON ECHINI. 



representing the Holectypina, but I have had no opportunity to study them. In these genera 

 low apophyses and auricles exist, and this structure with others affiliates the Holectypina with 

 the Centrechinoida (p. 218). 



In the Clypeastrina the perignathic girdle was described by Loven (1892) in representative 

 types. It presents striking ordinal characters. Apophyses are wanting, but auricles are 

 developed, and on these are inserted both the retractor and the protractor muscles, whereas in 

 regular Echini the protractors are always attached to the apophyses, or directly on the basi- 

 coronal interambulacral plates. As compasses are absent in the Clypeastrina, their muscles, 

 of course, are also absent. 



In Clypeaster subdepressus (text-figs. 231, 232) the ambulacral plates of the basicoronal 

 row roof over the primordial interambulacral plates so as nearly to cover them, but the inter- 

 ambulacral plates as far as visible are shaded, as seen in areas 2 and 3. The auricles are erect 

 pillars situated on either side of the ambulacrum, and, as the ambulacra arch over the inter- 

 ambulacra, therefore the auricles of two separate ambulacral areas come in juxtaposition, a 

 character seen only in this order. The four retractor muscles of each area are situated on the 

 inner and lateral upper faces of the auricles, and the two protractors, which are very small, 

 are situated on the bases of the same (text-fig. 231). Four retractors are not seen in all cly- 

 peastroids, and in other orders there are only two such muscles in each area. 



The degree to which the ambulacral plates roof over the primordial interambulacrals in 

 the basicoronal row varies in specimens and in different areas of the same individual. In 

 Clypeaster rosaceus (text-figs. 233-235) the beveling is considerable, but the whole length of 

 the interambulacral plate is seen from within, whereas in Clypeaster subdepressus (text-figs. 231, 

 232) the same plate is visible only at intervals. In this beveling of ambulacral plates it is to 

 be distinctly understood that we are considering the interior of the test. As viewed from the 

 exterior, it is a case of the interambulacra beveling over the ambulacrum, as discussed (p. 73). 



In Echinarachnius parma (text-fig. 236), as shown by Loven (1892, p. 67), a very remark- 

 able condition obtains; the auricles occur on the primordial interambulacral plates instead of 

 on the ambulacrals, and the small retractor and protractor muscles are inserted on the auricles. 

 In this type, therefore, the retractor and protractor muscles occupy the same interradial posi- 

 tion that they do in the Cidaroida, but instead of being inserted on apophyses, as in that 

 group, they are inserted on transposed auricles, a distinct morphological part. 



The case stated in brief is as follows : in the Echinocystoida and Perischoechinoida, as far 

 as known, no perignathic girdle existed, and the muscles were doubtless attached directly on 

 the base of the interambulacral plates. In the Cidaroida, apophyses are developed as upward 

 growths of the basicoronal interambulacral plates, and on these apophyses are inserted pro- 

 tractor, radial compass, and retractor muscles. In the Centrechinoida, apophj'ses are well or 

 feebly developed, and to them protractors and radial compass muscles are attached, but in this 



