SPONGES. 281 



CAMAROCLADIA DICHOTOMA U. & E. 



PL VII, Figs. 1. la. Ib. 



Sponge ramose, branching dichotomously at intervals of 10 

 mm. (more or less). Branches sub-cylindrical, 2 mm. or a little 

 more in diameter, with the surface smooth or minutely rough- 

 ened, and irregularly porous. Among the minute pores, which 

 we regard as interstices between the reticulate skeleton fibre, 

 a small number of larger ones may be detected. 



Sections show that the interior of the branches is mainly 

 occupied by more or less confluent short sub-cylindrical cavities, 

 with the separating partitions very thin. The cavities seem to 

 wind about in an irregular manner before they open at the 

 surface. Their width varies but little from 0.4 mm. 



The cluster of branches shown on plate VII, illustrates the 

 usual appearance of this peculiar sponge. The branches are 

 generally matted together and much broken in the rock or on 

 the surface of Antbaspidelhi species. In this condition the speci- 

 mens rarely show any recognizable structure. 



There are, probably, two or three other species of this genus 

 in the Trenton rocks of the western states. Though very 

 abundant in certain layers, the conditions of preservation were 

 in every case so unfavorable that none of the specimens pre- 

 serve more than the merest traces of the sponge skeleton. Two 

 of them are more robust than C. dichotomy, while the third is 

 a smaller form and its branches divide more frequently. 



Position and locality: Twenty-five feet above the base of the 

 Trenton limestone, near Dixon, 111. 



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