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



tubercles a little below the suture. The carina is double, formed by two rather remote 

 tubercled threads, the lower of which runs to the outer lip. Below this one is a broadish 

 furrow and slightly beaded thread, which, towards the mouth, projects so as to become a 

 third carina. The centre of the base has another slightly beaded thread ; and another, 

 formed of remote rounded tubercles, defines the umbilicus, within which is a very slight 

 furrow and an ill-defined ridge. Longitudinals — the apical whorls are ribbed, but the 

 ribs gradually break into the scarcely connected tubercles of the last whorl. The lines 

 of growth are hardly perceptible, except on the base. Colour pure white when weathered, 

 but apparently slightly brownish when fresh, with a pearly nacre below the thin calcareous 

 surface-layer. Spire high, very slightly scalar. Apex sharp, minute, flattened on the 

 one side, with the very small embryonic 1 J whorl rising sharply on the other. Whorls 7, 

 of regular increase ; the last is small, from the large part of it cut out by the umbilicus ; 

 they are flatly conical and slightly scalar. Suture linear, but strongly defined by 

 the right-angled junction of the whorls. Mouth oblique, much inclined to the axial 

 line, rectangularly rounded, the pillar and outer lip being parallel. Outer Up thin and 

 broken, not descending. Pillar-lip shortly but flatly bent over the umbilicus, and here 

 it is patulous and sinuated, it then advances in a straight line towards the base. It is 

 toothed in the middle by a strongish spiral protuberance, at which point it projects : but 

 from this to the junction with the base it is thin and retreats. Umbilicus very large, 

 funnel-shaped, and pervious. H. 0'27 in. B. 0"3, least 0'25. Penultimate whorl, (P08. 

 Mouth, height (PI, breadth 0-15. 



This very beautiful species is well defined by its exactly conical form and very large umbilicus, 

 whicb cuts the whole body out of the inside of the last whorl, narrowing the base to an extraordinary 

 extent. The young shells are excessively like those of Trochus {ZizvpMnus) tiara, Wats., but are 

 flatter, broader, and more umbilicated. 



Mr Dall (loc. cit. sup.) considers Margarita lamcllosa, Yerr. and Sin., to be this species. Dr Gwyn 

 Jeffreys again (Proc. Zool. Soc, 1883, p. 98) identifies with it Margarita rcgalis, Yerr. and Sm. 

 Both of these identifications can hardly be right, and I have therefore quoted the synonymy of neither 

 author. As to the identity of Margarita rcgalis, Yerr. and Sm., with Trochus ottoi, Phil., which 

 Dr Gwyn Jeffreys asserts, I express no opinion. So far as I have had opportunity of examining 

 specimens, they seem to me very fairly distinct, though offering resemblances suggestive of transitional 

 links. Mr Dall, who has had very many specimens before him, says that Trochus ceglees is very 

 variable, and this Dr Gwyn Jeffreys seems quite to confirm. 



48. Trochus {Margarita) clavatus,] Watson (PL V. fig. 8). 



Trochus (Margarita) clavatus, Watson, Prelim. Eeport, pt. 4, Journ. Linn. Soc. Lond., vol. xiv. 



p. 705. 



1 Clavus, a nail, with an allusion to the prickles which stud the shell. 



