GEODIA AGASSIZII. 



123 



near the end, or branched. Clades abruptly bent down at the end are repre- 

 sented on Plate 26, fig. 8, Plate 29, figs. 6, 15, Plate 34, figs. 5, 8. The ramified 

 clades (Plate 26, figs. 8-12; Plate 29, figs. 9, 10, 12, 14, 16, 17; Plate 34, fig. 15) 

 are so variable, that it is difficult to find two alike. Their branches either 

 extend in a longitudinal plane passing through the rhabdome, or less frequently 

 they diverge in different directions forming, if numerous, a terminal bunch. 

 They hardly ever he in the plane of the cladome and therefore differ fundament- 

 ally from dichotriaene-end clades. The simplest and most frequent forms of 

 these branched clades are those in which one straight, conical, thorn-like branch 

 arises from the lower (rhabdomal) side of the distal part of the clade. This 

 branch is either directed vertically downward (Plate 29, fig. 12), or, more fre- 

 quently, obliquely downwards and outwards (Plate 26, figs. 9-12; Plate 29, figs. 

 9, 16). Its size is in proportion to the distance of its origin from the end of the 

 clade; when it arises near the end of the clade it is small (Plate 29, fig. 16) 

 when it arises farther away from it, it is larger (Plate 29, fig. 9). In some 

 clades of this kind the branch is terminally divided into small secondary 

 branchlets (Plate 29, fig. 17). Sometimes the clades bear two simple or second- 

 arily ramified branches (Plate 29, fig. 10). The most complicated forms are 

 those in which the clade terminally divides into a greater number of divergent 

 simple, or more often, secondarily ramified branches (Plate 29, figs. 14, 16). 

 In the immature specimen, described by Lambe, orthoplagiotriaenes and dicho- 

 triaenes occur. According to Lambe (loc. cit., p. 37), the latter are much more 

 numerous than the former, "few examples of the simple orthotriaenes " being 

 found. I, on the contrary, found the orthoplagiotriaenes quite as numerous 

 as the dichotriaenes if not niore so. The orthoplagiotriaenes have a rhabd- 

 ome 2.1-3 mm. by 70-90 fi, and clades 300-450 /(long; the clade-angles are 

 91-103°. The dichotriaenes have a rhabdome 1-2.2 mm. by 50-75 /i, main 

 clades 150-300, and end clades 30-130 jj. long; the breadth of the whole 

 cladome is 350-700 /<, the main clades enclose angles of 109-112° with the 

 rhabdome. 



