and is closely allied to the preceding Cœlodendrida; they differ from the latter mainly in the development of a peculiar new organ, the "rhinocanna," or "nasal tube." This is a hollow tube placed in the sagittal plane, arising from the base of each galea, and is connected with its apex by a simple or double frenulum. Between the oral openings of the two opposed rhinocannæ (one dorsal and one ventral) lies the proboscis of the central capsule.
The first observed species of Cœlographida was Cœlographis gracillima, some parts of which (but not the entire skeleton) were figured in my Monograph (1862, Taf. xxxii. figs. 2, 3). But I confounded these with Cœlodendrum gracillimum, in the branched hollow trees of which the fragments of the former were entangled. I detected this error afterwards, when I had the opportunity of the observing some complete specimens. The first description of a complete skeleton was given in 1882 by O. Bütschli, who examined a large specimen of Cœlothamnus davidoffii, captured by Davidoff in the Mediterranean (Zeitschr. f. wiss. Zool., Bd. xxxvi. p. 486, Taf. xxxi.). In the rich collection of the Challenger I was able to distinguish not less than nine genera and twenty-six species of Cœlographida, but the majority of their large and most fragile skeletons were more or less injured, or quite broken. It was, therefore, of the highest importance for the minute study of this difficult group, that Dr. John Murray, during his expedition to the Færöe Channel (in 1882, in H.M.S. "Triton"), discovered in the Gulf Stream the beautiful Cœloplegma murrayanum, and brought up home hundreds of well-preserved specimens (Pl. 127). Only by the complete examination of this excellent material it was possible to answer many difficult questions as to their morphology, and to correct the errors in my description and in that of Bütschli.
We divide the family Cœlographida into two rather different subfamilies, which may afterwards be separated as two divergent families, the Cœlotholida (Pl. 122) and Cœloplegmida (Pls. 126-128). Both groups may be easily distinguished at a glance, since the numerous branches, arising from the hollow radial tubes, remain constantly free and independent in the former, and represent a spiny thicket, whilst in the latter they constantly become united, and by anastomosing form a peculiar "mantle," or outer envelope of delicate network. But besides, there are other and more important differences between the two groups. The peculiar hollow tube, arising from the base of the galea on each valve, which is filled with phæodella, and which we call the rhinocanna, develops in the Cœlotholida on its open mouth two paired lateral frenula (right and left), which connect it (like two lateral bridges) with the base of two paired hollow main tubes (the "frontal tubes"). In the Cœlospathida, however, the mouth of the rhinocanna develops a single odd frenulum only (in the sagittal plane), and is connected by it with the base of an odd, single, hollow main tube, directed forwards, the "nasal style."
The central bivalve lattice-shell, from which the galea and the tubes arise, exhibits in
