Parts of the Lantern of Aristotle in Echinus miliaris. 475 



the two fellow lamellae are found at the same level, inclined, at first, towards 

 each other at an angle which varies with their growth till, finally, as the 

 result probably of pressure from succeeding lamellae, their apices are brought 

 into contact and fuse together. 



At this stage of our investigation it is, perhaps, well to take into account 

 certain fundamental facts revealed to us : — 



(1) We see that a lamella does not pass through a tri-radiate spicule condi- 

 tion, unlike, in this respect, the other echinoderm ossicles which do, as far as 

 is known at present. 



(2) The growth of a lamella is unusual, being different from the normal 

 method of ossicle-formation which obtains among the other component 

 elements of the lantern. 



(3) There is a maximum limit to the vertical growth of a lamella, further 

 growth being along the two equal edges of each lamella. 



(4) From the fact that numerous pairs of lamellae go to build the adult 

 tooth, two conclusions can be drawn, viz., (a) that the urchin-tooth is a paired 

 structure, and (b) that a pair of lamellae is the unit of the same. This 

 admission of the morphological value of a pair of lamellae as a unit is not, as 

 will be seen later, prejudicial to regarding a cone formed by a pair of lamellae 

 as an integral structure. 



The process of formation of the tooth from these paired lamellae is not by 

 any means simple.* When two lamellae of a pair have attained their 

 maximum vertical growth, by the pressure probably of the succeeding lamellae 

 above, they become bent and concave to an equal extent on their inner 

 surfaces, their outer surfaces becoming correspondingly convex. During this 

 bending the two lamellae appear to grow along their lateral edges till those 

 of one lamella face each to each their two fellows on the other lamella 

 (text-fig. 1). This growth is maintained till the confronting edges meet and 

 fuse, forming a characteristic cone.f 



While the fusion of the inner edges is taking place, the rudiments of the 

 carina are found to arise. An inwardly directed flange appears at the top 

 corner of each inner edge (text-fig. 1). At about this time each cone under- 

 goes a flattening from the inner to the outer side. Partly as the result of 

 this flattening and partly owing to their growth, the above-mentioned flanges 

 project well towards the oesophageal side. They ultimately come into contact 



* Loven's (3, p. 9) description of the formation of a tooth, in Goniocidaris, from a 

 single row of lamellae is extraordinarily simple. He says as follows : " The lamels, 

 the general form of which corresponds with that of the tooth, are laid one upon another 

 regularly from the top downwards." Spencer (1904, p. 36), likewise, figures a simple 

 arrangement of lamellae in Palosodiscus fero.v. 



+ Theel (7) describes also a cone-in-cone arrangement in Echinocyamus pusillus. 



