478 MR. HERBERT L. HAWKINS ON 



the species Discoidea subucula, used the name Protocyamus " to 

 indicate the affinity of this Echinoid with the Echinocyamus 

 series." The name, on systematic grounds, must be abandoned, 

 but its significance remains. 



In Conulus, but, so far as I am aware, in that genus only, a 

 definite " sand-canal," similar to that of Echinoco7-ys and the 

 Spatangidse, is well developed on the inner surface of the madre- 

 poric genital. In the same genus, in adult specimens, a double 

 row of hemispherical prominences occurs, partly encircling the 

 inner part of the test a little above the ambitus. Klinghardt (68) 

 has recently discussed the relation of these thickenings to the 

 course of the alimentary canal, for the mesenteries of which they 

 seem to have given attachment. He has compared the coui'se of 

 the gut thus indicated with that of several fossil and recent species 

 of Spatangidae. In the present state of our knowledge, however, 

 but little of importance can be ascertained of the comparative 

 anatomy of the soft parts of fossil Echinoids. 



H. The Apical System. 



Much of the systematic work that has been done on the Irregular 

 Echinoids has had as its basis the character of the apical system. 

 Notable cases Avhere this feature has been utilized for the purposes 

 of classification are the works of Gaudry (' Enchainement du Monde 

 animal ') and Pomel. From a purely morphological standpoint the 

 system has Keen carefully described by Loven (31). In the case 

 of the Holectypoida, and of some of the near allies of that group, 

 I have recently summarized the state of our knowledge of the 

 apical system (Hawkins, 70) in a paper that was definitely a 

 preliminary note to the present work. In consequence, only the 

 comparative aspect of the subject need be dealt with here, and for 

 the description of details that paper may be consulted. 



The apical system is at first thoroughly disorganized in its 

 composition and structure by the passage of the periproct through 

 its cycle of plates. In Pygaster sens, str., the first stage of dis- 

 ruption is still visible. The posterior genital plate is entirely 

 absent, and the remaining four genitals are grouped in a roughly 

 semicircular order around the anterior edge of the periproct. The 

 madreporic genital is not much larger than the other three. 

 The oculars are small, and show no features of special importance. 

 From a broken and open condition such as that shov/n by Pygaster 

 sens, str., the processes of evolution wOrk along two definite 

 directions. The first aims at a restoration of a cyclic, or at least 

 of a compact, character in the system as a whole, and the second 

 is concerned with the infilling of the centre of the system (when 

 the cycle is regained) to replace the absent periproct. 



In the reconstruction of the cycle of genital plates, the posterior 

 (fifth) genital is not necessarily resuscitated. In fact, a very 

 large number of the great groups of the Irregular Echinoids are 

 permanently without this plate. Anorthopygus shows the simplest 



