SEA-URCHINS. 481 



fifth genital becomes practically indisting-uisliable, in point of size, 

 from the others of the cycle, even the right anterior plate 

 being much reduced from its condition in Ilolectypus. The pos- 

 terior plate may or may not be perforated, this irregularity 

 affording in itself ample proof of the plastic condition (in a 

 variational sense) of the genus. The madreporite, instead of 

 being restricted to the right anterior genital, is more or less 

 uniformly distributed over all five of these plates in some species, 

 a feature never found in the preceding genen). (In the case of 

 an otherwise abnormal Conulus albogalertts, a similar development 

 exists : see Hawkins, 70.) The oculars have dwindled considerably 

 in proportional size. 



The chief interest of this peculiar structure is seen when a 

 comparison is made between the apical systems of Discoklea and 

 Clypeaster. In the latter genus the madrepoi'ite is central 

 and prominent, but it is quite impossible to distinguish the 

 sutures of the genital plates, at least in adult forms. The 

 oculars are minute. Discoidea, then, shows the pi-eliminar}' 

 stages of the assimilation of the genitals — a phenomenon that is 

 preparatory to their coalescence and fusion in the Clypeastroida. 



Y. The Internal Evolution of the Order. 

 1 . F e a t u 1- e s of P h y 1 o g e n e t i c I m p o i- 1 a n c e. 



In pala}ontological attempts to trace a phylogenetic sequence 

 through any series of organisms, the first and essential feature to 

 be considered is the order in time in which the various forms 

 appear. Most of the serious errors that have marred the value 

 of some past work in this direction have resulted from an insufficient 

 I'eliance on the stratigrnphical i-elations of the genei-a considered. 

 It is true that our knowledge of the occui-rence of fossils at vai'ious 

 hoi'izons is very inadequate : it is only necessaiy to consider the 

 number of cases where a gap exists in the sequence of forms that 

 are known to occur in widely separated hoiizons, to I'ealize this 

 incompleteness of our knowledge. But it seems a fair postulate 

 to assume that the order in which various genei-a make their 

 appearance is approximately the true sequence of their evolution. 

 Especially is this the case in the Holectypoida. Not only are 

 they, in common with most Echinoidea, eminently adapted for 

 preservation in suitable deposits, but the periods of their exist- 

 ence, the Jurassic and the Cretaceous, were times wlien, at least 

 in this country, the conditions of deposition were exceptionally 

 favoural)le for the preservation of organic remains. In the scheme 

 of evolution put forward below, no apparent relationship has been 

 accepted unless the stratigraphical evidence confirmed it. 



A second great principle from which reliable evidence of genetic 

 affinity call be deduced is that of Ontogeny. Here, unfortunately, 

 our knowledge of the Holectypoida is meagre. In the Echinoidea 

 generally the process of recapitulation is always very much obscured 



Proc. Zool. Sue— 1912, No. XXXI. 31 



