11G2 ON TRIBRACHYOCRINUS CORRUGATUS, 



The intercostal or anal plate, which, as seen in our specimen, 

 presents a re-entering angle at its upper part, exhibits a bilateral 

 symmetry, being octagonal in shape, and is made to fit in the two 

 re-entering angles of the two preceding subradial plates. Lastly, 

 I will remark that the last row being composed as follows, three 

 radials, three interradials, and the second costal or second anal 

 p, forms a continuous set of plates fitting each other by alter- 

 nating re-entering angles. For instance, the second costal 

 occuping the space between an irregular radial and an inter - 

 radial, fits on one side, in the re-entering angle of the radial, and 

 on the other, is provided with a cuneiform projection fitting the 

 next interradial plate. 



Ornaments of the Calyx. 



The external ornaments of the Calyx are fairly impressed in 

 the external cast with which it was possible to obtain a positive 

 representation in plaster of Paris, of the outer part of the Calyx. 

 These ornaments are composed of coarse granulations which give 

 the fossil an apparent resemblance to Platycrinus granulatus 

 (Austin) of the Carboniferous of Belgium. The difference, how- 

 ever, is very great, between the ornaments of Platycrinus granu- 

 latus and those of Tribrachyocrinus corrugatus. In the first they 

 consist of irregular tubercles, sometimes following each other in 

 sequence or meeting together for a short distance, but without 

 regularity. Sometimes these tubercles are rounded, sometimes 

 they are angu lar 



In Trib. corrugatus the ornaments form a network of ridges, 

 leaving hollows between them, except on the radials where 

 separated tubercles disposed into radiating lines are to be seen 

 as in fig. 7, pi. 68. 



Both the external cast and the internal cast are represented. 

 PI. 68, figs. 2 to 5. 



These internal and external casts show, above the three radials, 

 the impressions of the second radials which were not known 

 before. These I was more inclined to call first brachial articles ; 

 they come into contact with the radials by a sharp straight edge, 

 their under surface, as well as the surface of the corresponding 

 part of the radial, exhibiting fine strioo produced by the attach- 

 ment of muscles which allowed the arms to move in a plane 



