122 



GEODIA AGASSIZII. 



In young orthoplagiotriaenes the entire clade, in the fully developed ones 

 its basal part only, is directed obliquely upward. In their further course the 

 clades of the fully developed orthoplagiotriaenes bend downwards, so that their 

 distal parts lie more or less in a plane vertical to the rhabdome. The chords of 

 the clades of the orthoplagiotriaenes of the adult specimens enclose angles of 73- 

 117°, on an average 98.2°, with the axis of the rhabdome. In the adult speci- 

 mens from Stations 3168 and 4228 the clade-rhabdome angles do not exceed 

 100°, so that here all these triaenes appear as orthotriaenes. In the adult 

 specimens from the seven other stations a smaller or a greater number of such 

 triaenes with clade-chord angles exceeding 100° and appearing as plagiotriaenes 

 in consequence, are observed. The average clade-angle, however, exceeds 100° 

 only in the specimens from Stations 2886, 2887, and 4193. The angles of the 

 three clades of the same cladome are usually nearly equal. It is very rarely that 

 they become so different as to give the cladome a position oblique to the rhabd- 

 ome. Such orthotriaenes with oblique cladomes are represented on Plate 26, 

 fig. 3, and Plate 29, fig. 4. 



The size and the shape of the clades are far from constant, not only the 

 clades of different orthoplagiotriaenes of the same specimen but even the clades 

 of one and the same triaene often being very dissimilar. Their maximum aver- 

 age dimensions are, however, about the same in all the specimens. At the base 

 the clades are a little thinner than the cladomal end of the rhabdome, the ratio 

 between these two dimensions varying between 7 to 10 and 9 to 10. The clades 

 are 240-560 fx long, their average maximum length being 490.89 /«. The 

 maximum average of those of the orthoplagiotriaenes of the specimen from Sta- 

 tion 4551 is the smallest, of those of the orthoplagiotriaenes of the specimens 

 from Station 4199 the largest. The orthoplagiotriaenes of the young speci- 

 men from Station 4228 have clades 300-500 /x long, their average maximum length 

 being 490 ix. Their chords enclose angles of 88-108°, on an average 97° with 

 the axis of the rhabdome. 



In all specimens orthoplagiotriaenes with simple clades, gradually de- 

 creasing in thickness and curvature towards the usually not very sharp-pointed 

 end (Plate 26, figs. 3, 4, 6, Plate 29, figs. 1-5, 8, 11), are met with. In the speci- 

 mens from six of the stations all the orthoplagiotriaenes, or at least a very great 

 majority of them, have regular clades of this description. In the specimens 

 from Stations 3168, 4193, and 4199 on the other hand the orthoplagiotriaenes 

 with such simple and regular clades are not so numerous as orthoplagiotriaenes 

 with one or more clades rendered irregular by being either abruptly bent down 



