BY F. RATTE, ENG. ARTS AND MANUF., PARIS. 1161 



anal, and the adjoining radial and interradial (A + AB), and 

 (B + A B), can be called the Anal region, its plane axis cutting 

 in halves (A B) and p. 



The detail ol this arrangement is the following : — 



The tripartite division of the basal plates and the situation of 

 the so-called anal plates cause the row of five plates which follow 

 the basal and are called sub-radial (sous-radiales) by Prof, de 

 Koninck, and first costals by Prof. McCoy, to be, necessarily, 

 formed of irregular elements. 



Prof, de Koninck, at page 161 has given a geometrical diagram 

 of the plates, of Tribrachyocrinus Clarkei. The basal pentagon 

 in this diagram is made regular, and the three sides on which 

 fall the divisions are made straight. The diagram given by 

 Prof. M'Coy of the same species is nearer the diagram I give of 

 the new species (pi. 68.) 



The fossil being observed from above, the medial line of division 

 of the basal plate projected downwards and the two lateral lines 

 of division projected upwards, it will be seen that the basal 

 pentagon is not regular, and may even be more exactly considered 

 as an irregular octagon with three re-entering angles at the 

 points of junction of the three segments, the general outline of 

 the figure, however, approaching a regular pentagon. Moreover 

 of the two segments adjacent to the medial division a i, one much 

 more extended than the other, is the segment adjacent to the anal 

 region, and, as a consequence, the angle a i d is greater than the 

 angle a if. 



To follow this first irregularity, the three subradial plates which 

 are not adjacent to the anal region, are not of the same shape, 

 one C, adjacent to b c, is pentagonal, whilst the two D and E, 

 adjacent to cde, and efg, are, we may say, hexagonal with two of 

 their sides only about half the length of the others. 



As to the two other subradial plates A and B, those adjoining 

 the anal region, they differ only a little from each other. One 

 of them, B, adjacent to hab being irregularly octagonal, whilst 

 the other one, A, adjacent to gh, is irregularly heptagonal, both 

 with one re-entering angle. 



