90 ROBERT TRACY JACKSON ON ECHINI. 



Genital plates may be divided by secondary sutures as later discussed, but oculars are 

 very rarely divided. A case, however, is shown in text-fig. 193, p. 169, in which dividing 

 sutures develop in ocular plates, as also seen in Plate 6, fig. 6. 



Having seen the characters of ocular and genital plates in their mutual relation in the 

 Palaeozoic, it is of much interest to compare those of later times. Little attention has been 

 directed to these plates, but a close study reveals characters of importance to general mor- 

 phology, to the evolution of the group, to the relation of the species in the genus and related 

 genera, and to geographical distribution. Ocular plates present an excellent systematic char- 

 acter which has been largely overlooked. Osborn (1901) in a brief but very suggestive paper 

 called attention to the variations of ocular plates in Arbacia and compared these variations 

 with the characters of Palaeozoic types. 



Early in my studies of these plates it was seen that they had an important bearing, and 

 observations were made on all available specimens of regular Echini, Mesozoic and Recent. 

 In the fossils this is not always easy, as for purposes of study, all five oculars and genitals must 

 be observed, and they are frequently lost in fossils. I have succeeded, however, in making 

 observations on something over 50,000 regular Recent and Mesozoic specimens representing 

 133 species. The results are presented in tabular form and the more interesting cases are 

 described. In the tables in certain species, developing series are given as well as developed 

 series, notably in Strongylocenlrotus drobachietisis ; in other species the specimens listed were 

 not all adults, but were all large enough so that they were believed to be fully developed, being 

 for the most part nearly or quite half grown. Of the species tabulated all specimens seen are 

 included excepting young specimens, those without locality, where this was a feature of 

 importance, tetramerous or hexamerous individuals, and a very few distorted specimens that 

 could not properly be included. The reason for making so many observations was that while 

 the character of a species is usually gathered correctly from five or ten specimens, the varia- 

 tions seen in a large number present interesting data for comparative study. 



One may expect to find the common variants in from 25 to 100 specimens, and 500 to 1,000 

 may reasonably be expected to yield all the variants at all likely to occur. For an exhaustive 

 study of variation, however, many thousand specimens of a species may be profitably studied. 



In Mesozoic regular Echini the dominant character is for all the oculars to be exsert, or 

 excluded from the periproct. In the Recent regular Echini the young also have all the oculars 

 exsert.^ In the adult all the oculars may be exsert or one or more may be insert. Wliile the 

 exsert character of the young is like the Mesozoic, the becoming insert in development is the 

 taking on of a character which in this respect is directly comparable to the dominant character 

 of the Palaeozoic. 



' By young is meant specimens of from 2 to 5 or more millimeters in diameter. At an extremely early stage, at least as 

 shown in Echinus, Plate 3, fig. 5, all the oculars reach the periproct. A general conception of the characters of this stage 

 is represented schematically in Plate 3, fig. 7. 



