146 Papers from the Marine Biological Laboratory at Tortugas. 



chinus when oculars begin to enter the periproct, the first to enter is V or I, 

 but in the Montego Bay series V is by far the more common/ they being in 

 the ratio of 102 of ocular V to 23 of ocular I. In this feature Centrechinus 

 makes an approach to the Cidaridae, in which group, when one ocular is 

 insert, it is almost always ocular V. As previously shown, when in the 

 Centrechinoida four oculars reach the periproct, it is typically I, V, IV, II, 

 and when any other combination exists it is nearly always I, V, IV, III. 

 This combination, while rare in the Centrechinoida as a whole, is rela- 

 tively more frequent in Centrechinus and is a typical feature or a frequent 

 variant in some species of the Cidaridae. In this feature also, therefore, 

 Centrechinus makes a certain approach to the Cidaroida. 



In Centrechinus, when opened fresh and alive, it is found that the teeth 

 extend above the base of the foramen magnum about to the upper line of 

 the lantern, but the proximal base of the teeth does not extend horizontally 

 over the lantern as in the Echinidse, Strongylocentrotidae, and Echinome- 

 trldse. Further, the dental capsule, inclosing the base of the tooth, is small. 

 In the limits of the teeth dorsally and the small dental capsule, as well as 

 in the grooved teeth, Centrechinus makes a close approach to the characters 

 of the Cidaridae as represented by Eucidaris trihuloides. 



M 



3 



Figs. 2-6. — Perignathic girdle in Aspidodiadema and its development in Centrechinus. 

 2. — Adult Aspidodiadema meijerei (Doderlein), Pailolo channel, Hawaiian Islands. The auricles are 



separate spur-like styles. 

 3. — Centrechinus setosus (Leske), Montego Bay, Jamaica. Specimen 5 mm. diameter. Auricles are 



separate styles, apophyses faint. 

 4- — The same. Specimen 10 mm. diameter. Auricles are more developed and arch over the ambulacra. 

 S. — The same. Specimen 25 mm. diameter. Auricles meet over the ambulacra in a slender arch. 

 6. — The same. Specimen 70 mm. diameter. Auricles developing a high plate-like cliaracter, apophyses 



also are high. 

 7. — Centrechinus setosus, Bermuda. Specimen no mm. diameter. Auricles have developed a high 



plate-like character; apophyses are high ridges. 



Very little attention has been paid to the development of the perignathic 

 girdle excepting by Loven, who described it in Cidaris and Strongylocentrotus 

 (Echinologica) , I also have published a few observations on the develop- 

 ment and variation of the perignathic girdle. Centrechinus, while being a 

 primitive type in many respects, has in the adult a specialized perignathic 

 girdle having high auricles which meet in a broad plate-like arch over the 

 ambulacra and also high apophyses on the interambulacral areas (fig. 7). 

 Such being the character of the adult, it is of much interest to follow the 

 development of this, which is one of the most specialized perignathic girdles 

 known in Echini. When a specimen is 5 mm. in diameter, the apophyses 

 show only slight indications of development and the auricles stand up as 

 separate, blunt, spur-like processes on either side of the ambulacrum (fig. 3) . 



1 In previous observations from various localities with limited material, ocular I predominated over V 

 numencally. 



