GEODIA AGASSIZII. 121 



The orihoplagiotriaenes (Plate 26, figs. 3-12; Plate 29, figs. 1-6, 8-12, 14-17; 

 Plate 34, figs. 1-12, 14, 15) occur in large numbers in all specimens. Their 

 rhabdomes are straight or slightly curved and usually regularly conic, occasionally 

 more cylindrical. The conical rhabdomes are generally pointed (Plate 26, figs. 3, 

 5-12; Plate 34, figs. 1-7), more rarely blunt (Plate 26, fig. 4; Plate 34, fig. 9). 

 The more cylindrical rhabdomes are more or less shortened and thickened to 

 tyles at the acladomal end (Plate 32, figs. 12b; Plate 34, fig. 8, 10-12, 14, 15). 

 Orthoplagiotriaenes with conical rhabdomes occur in all the specimens. The 

 orthoplagiotriaenes with more cylindrical, shortened and terminally thickened 

 rhabdomes are exceedingly rare in the adult specimens, but quite numerous in 

 the young specimen from Station 4228. The ordinary conical orthoplagio- 

 triaene-rhabdomes of the adult specimens are 1.5-4.2 mm. long and 65-150 n 

 thick at the cladome, their average maximum dimensions being 3.39 mm. X 

 115.44 /ii. In the ordinary orthoplagiotriaenes of the specimen from Station 

 4551 the rhabdome is remarkably stout, shorter and at the same time thicker than 

 in the ordinary orthoplagiotriaenes of any of the others. In the ordinary ortho- 

 plagiotriaenes of the specimens from Station 4193 on the other hand the rhabd- 

 ome is very slender, its length being above the mean and its thickness less than 

 in the orthoplagiotriaenes of any of the othere. The conical rhabdomes of the 

 orthoplagiotriaenes of the young specimen from Station 4228 are 2-3.5 mm. long 

 and at the cladome 50-100 fi thick, their average maximum dimensions being 

 3.3 mm. X 90 /«. The orthoplagiotriaene-rhabdomes reduced in length and 

 terminally thickened (Plate 34, figs. 8, 10-12, 14, 15) are more cylindrical, and 

 much less attenuated towards the distal end than the ordinary ones. The 

 degree of this attenuation is proportional to the length, very short ones (Plate 

 34, fig. 15) being not attenuated at all and regularly cylindrical. The terminal 

 thickening is usually fairly spherical. Its diameter is 15-25 % greater than 

 the thickness of the part of the rhabdome lying just above it. Sometimes one 

 or more slight thickenings of the rhabdome are observed above the terminal 

 thickening. A cyUndrical, terminally thickened orthoplagiotriaene-rhabdome 

 of an adult specimen from Station 2978 was 900 /x long and 155 /z thick at the 

 cladomal end, the thickness of the thickened acladomal end being 170 /t. In the 

 young specimen from Station 4228 these more cylindrical orthoplagiotriaene- 

 rhabdomes are 760 /x-2.35 mm. long, and at the cladome 60-105 /« thick. Their 

 thickness is, roughly speaking, in inverse proportion to their length. The two 

 shortest observed, one of which is represented on Plate 34, fig. 15, were 950 and 

 760 p. long and 95 and 105 /i thick respectively. 



