Studies of Jamaica Echini. l6i 



" Phylogeny " a detailed discussion is given of the bearing of aberrant variants 

 which, although aberrant, follow quite definite lines of their own. Of the 

 total 51,419 specimens considered above, 50,591 , or 98.39 per cent, are correct 

 by rule as regards ocular arrangement, and 828 specimens (of which 360 are 

 I, V, II as above noted), or 1.61 per cent, have an aberrant arrangement. 



From the above records it is seen that ocular plates in the Centrechinoida 

 follow a very definite line as regards which ones reach the periproct, and by 

 their arrangement emphasize a bilateral symmetry through the axis of 

 ambulacrum and ocular III and interambulacrum and genital 5. This is 

 the axis on which the irregular types develop an elongate form, eccentric 

 periproct and other features of bilaterality, and it was urged by Loven as 

 the correct axis for orienting Echini. Mr. A. Agassiz {Challenger Echini, 

 pp. 4-6) criticizes Loven's determination of the antero-posterior axis in 

 Echini. His argument, however, simply shows that he misunderstood 

 Loven's point of view. 



In accordance with what I previously showed in Strongylocentrotus it is 

 seen that the perforation of the genital plates by the genital pores originates 

 at quite definite periods. In Centrechinus setosus the genitals are imper- 

 forate in specimens up to 14 mm. in diameter, after which with few ex- 

 ceptions all specimens have developed the genital pores. In Echinometra, 

 genitals are imperforate up to 10 mm., after which all plates are perforate. 

 Both of these are later in developing than Strongylocentrotus drobachiensis, 

 which is imperforate only up to 5 mm. in diameter, after which in most 

 cases the genitals are perforate. 



In Eucidaris tribuloides and Centrechinus setosus the teeth extend to 

 the upper line of the lantern, but do not extend horizontally over the top 

 of the same; also, in both the dental capsules embracing the base of the 

 teeth are small. These are both doubtless primitive conditions, and are 

 opposed to the condition found in the Echinidse and Strongylocentrotidae. 

 In Toxopneustes variegatus, Tripneustes esculentus, and Echinometra lucunter 

 the teeth extend horizontally over the top of the lantern, the proximal end 

 of the tooth is grooved, not keeled, the keel appearing further down the 

 tooth, all as in Strongylocentrotus. This grooved character is of interest as 

 a localized stage in development, as it shows that the young growing point 

 of the tooth has the character retained throughout life in the Aulodonta, 

 Cidaroida, and Palaeozoic genera, in which the teeth are always grooved. 

 In Toxopneustes, Tripneustes, and Echinometra the dental capsules are 

 large, here again being like the character of Strongylocentrotus and diff'ering 

 from the condition in Eucidaris and Centrechinus. 



In Centrechinus setosus the perignathic girdle is specialized in the adult, 

 with high apophyses and high laminar auricles joined over the ambulacra. 

 In the young, the apophyses are very slight and the auricles are simple 

 spur-like processes, as in the adult Aspidodiadematidae, which represent 

 the simplest auricles known. Later, in Centrechinus, the auricles arch over 

 the ambulacra, but without fusion. Then they fuse in delicate arches, a 



