ARISTOTLE'S LANTERN AND PERIGNATHIC GIRDLE. 187 



opneustes, teeth keeled, foramen magnum open, epiphyses narrow, and spurs from the produced 

 dorsal tips of the pyramids supporting the teeth. These characters link these two families 

 together and equally separate them from other families of regular Echini. 



In Arbacia, as in Salenia, the teeth are keeled (text-fig. 212). The pyramids have a rather 

 deep foramen magnum, and the upper faces of the half-pyramids, seen when the epiphyses are 

 removed, show a pitted surface. The epiphyses are narrow, little more than capping the half- 

 pyramids, so that the foramen magnum is open dorsally. The braces of Arbacia are of the 

 usual type, but the compasses are distally rounded instead of bifid as in most Echini. This 

 feature was found in Arbacia lixula, A. pundulata, and A. nigra, so that it is probably generic. 



The Stirodonta as regards the lantern present features intermediate in character, having 

 narrow epiphyses as in the Aulodonta and still more primitive types, but keeled teeth like the 

 Camarodonta. Keeled teeth are evidently mechanically more powerful than grooved teeth 

 (text-figs. 207-212, p. 184). Therefore this structure can be considered a mechanical as well 

 as a structural advance over what obtains in the lower groups of Echini. 



Camarodonta. — Tripneustes represents the most complex structure known in the Centre- 

 chinoida. The teeth are keeled (text-fig. 213), and are supported by the dental slides. The 

 epiphyses are wide and meet in a median suture over the foramen magnum. In addition, the 

 epiphyses bear crests, which are strongly developed in Tripneustes, and these processes bear 

 against and give support to the teeth at their upper portion. The pyramids are nearly per- 

 pendicular and have a deep foramen magnum, which is roofed over by the joined epiphyses. 

 The dorsal face of the half-pyramids, seen when the epiphyses are removed, shows the pits 

 characteristic of the order. The styloid processes are strongly developed, as seen in face view. 

 The epiphyses have the usual glenoid cavity and tubercles for articulation with the brace. 

 The brace is of the usual character, and the ossicles of the compass are divided into two parts 

 and are distally cleft as usual. Strongylocentrotus as previously described (Plate 5) is essen- 

 tially like Tripneustes. A peculiar variation was found in a specimen of Tripneustes esculentus 

 from Bermuda. In this there are no jiits in the top of the pyramids, and as such it may be 

 considered a regressive variant. It is tlie only case seen in the Centrechinoida in which these 

 characteristic pits are wanting. 



The character of the Camarodonta as described is to have keeled teeth and wide epiphyses 

 meeting in suture over the foramen magnum ; also the epiphyses bear crests which give support 

 to the teeth. This is the feature of the Temnopleuridae as ascertained in Temnopleurus, 

 Microcyphus, and Amblypneustes. It is the character of the Echinidae as ascertained in 

 Echinus, Toxopneustes, and Tripneustes, and of the Strongylocentrotidae, as seen in Sphaere- 

 chinus and Strongylocentrotus. Finally, it is the character of the Echinometridae, as ascertained 

 in Echinometra and shown by Mr. Agassiz (1908) in Colobocentrotus. The lantern of the 

 Camarodonta is mechanically as well as structurally the most highly evolved known in Echini. 



