734 THE VOYAGE OF H.M.S. CHALLENGEK. 



is commonly truncated or liroken off, sometimes pyramidal. The central end is every- 

 where thinned, more or less pyramidal, and the neighbouring spines are propped one 

 upon another by the triangular faces of their small basal pyramids. A slight pressure 

 is sufficient to destroy their connection. 



The number and disposition of the radial sj^ines seem to be variable and irregular, 

 but require further researches. In four of the observed eight S2>ecies I found constantly 

 ten spines, in two other species from ten to twenty (commonly twelve or sixteen), 

 and in two species twenty or more. A certain order or disposition of the spines within 

 the conical space in which they radiate could now*here be ascertained. 



When I first observed Litholophus, I supposed that it might only be a mutUated or 

 altered form of an Acanthoma. Afterwards, observing many sjiecimens with ten 

 spines, I was led to the suggestion that they were produced by self-division of an 

 Acanthoma, and that the number of the spines in each half of the body might be after- 

 wards doubled. But this suggestion seems to be refuted by the fact that in no other 

 genus of the numerous Acantharia is self-division observed, and that many hundreds 

 of Litholophus which I observed exhibit quite constantly only a single form of radial 

 spines, that of Acanthoma — simple quadrangular spines without any apophyses. 



Genus 320. Litholophus,^ Haeckel, 1862, Monogr. d. Radiol., p. 401. 



Definition. — L it hoi ophida with a variable number of quadrangular diverging 

 radial spines, united with pyramidal bases in the apex of the conical central capsule. 



The genus Litholophus, the only one of this family, exhibits the peculiarities just 

 described, but might more nearly be defined as a t)'pical " genus " by the quadrangular 

 form of the radial spines, identical with those of Acanthoma. 



The central capsule of Litholoj^hus is constantly conical or pyramidal, commonly 

 opaque, of a dark brownish or reddish colour; it contains many small nuclei. It 

 envelops the basal half of all radial spines in such a manner that their* basal parts are 

 united in its apex, and their distal parts pierce the rounded base of the conical capsule 

 (PI. 129, fig. 2). 



The calymma or the jelly envelope of the central capsule is only developed at its 

 base, where the sj)ines radiate; at the conical mantle of the capsule it is very thin. The 

 spines seem to be perfectly enclosed in the calymma and connected with it by the same 

 contractile retinacula or " myophrisca" which we observe in the Acanthonida. The 

 pseudopodia arise only from the rounded base of the conical capsule, and radiate between 

 the spines, piercing the calymma, diverging within the conical sj^ace occupied by the 

 fascicle of spines. 



1 LtiAoJopAus = Stony brush ; xWo;, 'h-iifo;. 



