ARISTOTLK'S I.AXTKKX AND I'KUKJNATIIK MitiH.K. 187 



opneustes, teeth keeled, foramen magnum open, epiphyses n.-irn.w, .-mil -pur- fn.m tin- produced 

 dorsal tips of the pyramids supporting II,,. teeth. TbCM character! link tl. i fam.he- 



together and equally separate them from other familie.- ..f regular Kdiini. 



In Arbacia, as in Salenia, the teeth arc keeled (text-fin. -'I'.'i. Th- pyramid- have a rather 

 deep foramen magnum, and the upper fares <,f the half-pyramids seen when the epipl 

 removed, show a pitted surface. The epiphyses are narrow, little mure than rapping 1 1..- half- 

 pyramids, SO that the foramen magnum is open dorsally. The brace-, of Arl.acia an- ..f the 

 usual type, but the compasses are distally rounded instead of bifi.l a- in mo-t Kchini. Tin- 

 feature was found in Arbacia lixula, A. pundulata, and A. liigrn. .-o that it i.- pn.babh generic. 



The Stirodonta as regards the lantern present features intermediate in rharaeter, 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 a> well 

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



Camarodonta. Tripneustes represents the most complex structure known in the ('entre- 

 chinoida. The teeth are keeled (text-fig. 213), and are supported by the dental slide-. 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 the-e p< 

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

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

 The dorsal face of the half-pyramids, seen when the epiphyse.- are removed, -ho\\s the pits 

 characteristic of the order. The styloid processes are strongly developed, a- seen in fare view. 

 The epiphyses have the usual glenoid cavity and tubercles for articulation with the ]> 

 The brace is of the usual character, and the ossicles of the coinpa-s are divided into two p 

 and are distally cleft as usual. Strongylocentrotus as previously described il'lat' 

 tially like Tripneustes. A peculiar variation was found in a specimen of Tripnetutet <-xcnl. 

 from Bermuda. In this there are no pits in the top of the pyramids, and a- Midi it may be 

 considered a regressive variant. It is the only case seen in the Centrechmotda in which th. -e 

 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 cn-ts 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 Strongyloeentn>tid:i' D in Sph 



chinusand Strongylocentrotus. Finally, it is the character of the Echinoinctridae, as ascertained 

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

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



