ARISTOTLE'S LANTERN AND PERIGNATHIC GIRDLE. 179 



An epiphysis surmounts each half-pyramid to which it is joined by close suture (Plate 5, 

 figs. 2-7, 9). The epiphysis extends proximally, capping the half-pyramid, and laterally ex- 

 tends over the foramen magnum and meets its fellow of the opposite side in a median suture. 

 Each epiphysis presents a glenoid cavity and an internal and an external tubercle (Plate 5, 

 figs. 3, 5, 9), which serve in articulation with the brace. It also bears an elevated process or 

 crest which is apposed to the tooth and dorsally supports that organ. The extension of the 

 epiphyses over the foramen magnum so as to unite in suture and the crests developed on the 

 same, ai-e important characters seen in tlie four families of the Temnopleuridae, Echinidae, 

 Strongylocentrotidae, and Echinometridae, comprising the new suborder Camarodonta, but 

 are absent in other Echini (p. 183). 



The brace is a block-shaped plate, often called rotula, which rests on and interlocks with 

 the two opposed epiphyses (Plate 5, figs. 9, 11, 12). On its outer lateral borders it presents 

 two condyles which fit into the glenoid cavities of the epiphyses. The compass (Plate 5, figs. 

 2, 9) rests on top of the brace, extends over its whole length, is attached to the brace on its 

 inner proximal end by a tiny ligament, and is bifurcated on its outer end. Each compass 

 consists of two parts, a suture just within the circular compass muscles separating the compass 

 into an inner and an outer piece. The structure of the compass and brace is quite uniform 

 in Echini, excepting the clypeastroids. There are thus forty pieces in the lantern of Strongylo- 

 centrotus, and in all other Echini that possess a lantern, excepting the clypeastroids. These 

 parts are in brief : five teeth ; ten half-pyramids ; ten epiphyses ; five braces ; ten pieces making 

 up the five compasses. 



Next taking up the soft parts associated with the lantern of Strongylocentrotus drdbachiensis, 

 we find that each tooth at its basal poi'tion is enclosed in a voluminous, very delicate trans- 

 parent sac that may be called the dental capsule. A dental capsule is figured by the Sarasins 

 (1888) in Asthenosoma,' and is mentioned in Cidaris by T. H. Stewart (1861) and in Echinus by 

 Chadwick (1900), but I think it has not been adequately shown before. In order to see the 

 sac properly, a specimen should be opened alive, right out of the sea. It is so delicate and sensi- 

 tive, that if opened a few hours later, though the animal be still active, the sac has shrunk to 

 small proportions. In alcoholic material as far as observed it is always collapsed. When 

 opened thus alive, the capsules are so inflated that they look like five bladders radially arranged, 

 and so large that they actually touch one another at the area nearest the oesophagus (Plate 5, 

 fig. 1). The sac envelops the base of the tooth completely, lies free, but on its outer border 

 passes over the epiphyses and some distance down the face of the area of the foramen magnum 

 (Plate 5, fig. 6). Similar capsules were seen in fresh specimens of Arbacia ipunctulata, Sphaer- 

 echinus granularis, and Strongylocentrotus lividus, and very small ones in Echinarachnius parma. 

 It would be interesting to study the capsules in perfectly fresh material of other Echini. 



' Unfortunately in Lang's (1896) copy of this figure, tlie dental capsule is labeled Polian vesicle. 



