1714 THE VOYAGE OF H.M.S. CHALLENGER. 



two aboral spines are developed, the caudal horns (a dorsal and a ventral) ; these are 

 very large in Conchoceras (PI. 124, figs. 15, 16). But a peculiar and most interesting 

 kind of junction is effected in some Concharida by a true ligament between the valves 

 (PL 123, figs. 8, 9 ; PL 125, fig. 2). This ligament is always placed on the aboral 

 hinge, is of dark brown colour, and is not dissolved by mineral acids unless long apphed. 

 It may preserve the connection of the posterior parts of both valves, when their anterior 

 parts are removed one from another, just as in the Brachiopoda. I observed this 

 interesting ligament mainly in the genus Conchopsis, but not in all species, and it 

 is not yet certain whether it is a constant organ in these and some other Concharida. 



In the majority of Concharida the lateral margins of the two valves project slightly 

 inwards into the cavity, and in some species of Conchopsis these inner borders are so 

 broadened that they form a broad, horizontal, fenestrated inner shelf, comparable to 

 the deck of a boat or to the velum of the Hydromedusse or Craspedotse (PL 125, 

 fig. 9). In this case the velum surrounds the ovate aperture through which the two 

 lobes of the central capsule (dorsal and ventral) enter into the cavity of both valves. 



The mouth of the shell lies on the oral pole of the main axis, and is therefore 

 opposed to the aboral hinge. The two valves are here usually more or less emarginate, 

 so as to form a transverse mouth with an upper and a lower lip (PL 124, figs. 6, 7, 11). 

 The form of these two lips is often very difterent and characteristic of particular species 

 (PL 124, figs. 3, 15, 16). The mouth remains in many species constantly open, even 

 -when the frontal fissure is closed (figs. 7, 16). Since the centre of the shell mouth lies 

 in the prolongation of the proboscis arising from the operculum of the central capsule, 

 probably the main stream of sarcode, issuing from the latter, becomes protruded by the 

 former. 



Apophyses of the shell (besides the teeth of the margins) are completely wanting 

 in three genera, Concharium, Conchelliuvi, and Conchopsis (PL 123, figs. 1—4, 7; 

 PL 125). The four other genera possess free apophyses or spines, which we call 

 horns. They are probably important as the beginnings of those large hollow 

 tubes which are characteristic of the two following families, Coelodendrida and 

 Coelographida. We distinguish two difi"erent forms of horns, apical horns on the poles 

 of the sagittal axis, and caudal horns on the aboral pole of the main axis ; the former 

 probably correspond to the sagittal tubes and the latter to the caudal tubes of the two 

 following families. Apical horns are found in a single genus only, Conchonia 

 (PL 124, figs. 10-14). Here either on one pole or on both poles of the sagittal axis 

 a horn is developed, usually curved backwards. Sometimes the base of this conical 

 horn is inflated and fenestrated, and may represent the beginning of the formation 

 of the galea or apical cupola of the Coelodendrida. 



The two caudal horns are opposite on the aboral hinge of the shell, one arising from 

 the posterior end of each valve. Usually they are short and thick, pyramidal, the 



