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



than the eighteen other spines. In my Monograph (1862, p. 389) I had united all 

 these Acanthonida in a single genus AmphUonclie (with ten species). Some species 

 of it are very common and widely distributed ; but in general the number of different 

 forms in this family is much smaller than in the two foregoing families. 



The two principal spines, which in all Amphilonchida are much larger than the 

 eighteen other spines, characterise the " hydro tomical axis " or the larger equatorial axis. 

 The two other equatorial spines or the " geotomical spines " are much smaller, and 

 commonly of the same size as the eight tropical and the eight polar spines. In the 

 genera Amphilonche and Amphibelone these eighteen smaller spines are rather equally 

 developed; sometimes they are very small or cpiite rudimentary, so that the skeleton 

 seems to be represented only by the two very long principal spines (PL 132, fig. 4). 

 The genus Acantholonche is distinguished by the unequal size of the eight tropical and 

 the eight polar sjiines, the latter being more or less rudimentary. However, the 

 central bases of all twenty spines, by which they are united in the centre, are constantly 

 present. The genus Amphihdone is distinguished by the unequal size and form of the 

 two principal spines, one of them, the " caudal spine," being larger (and often of another 

 form) than the opposite " frontal spine." 



The form of the radial spines is in the Amphilonchida far less varied and complicated 

 than in the other Acanthonida (the Astrolonchida and Quadrilonchida). Apophyses or 

 lateral transverse processes are never developed. The three main forms of spines are 

 the same as in the other Acanthonida; they are (l) either cylindrical or conical (like 

 Acanthometron), or (2) compressed or two-edged (like Zygacantha), or (3) quadrangular 

 or four-edged (like Acanthoma). Often the spines are angular in the inner or proximal, 

 roundish in the outer or distal part. The distal apex is commonly simple, conical or 

 pyramidal. The central base is commonly also pyramidal, as in the majority of the 

 Acanthonida ; and the triangular faces of the neighbouring bases are simply propped 

 one upon another. More rarely a basal leaf-cross is developed above the basal pyramid. 

 Very rarely the central bases of the united spines grow together in the centre, so 

 that the whole skeleton forms a single piece of acanthin. 



Hie Central Caj>sule is rarely spherical, commonly prolonged in the direction of the 

 hydrotomical axis ; ellipsoidal or cylindrical, sometimes also four-sided prismatic ; it 

 commonly envelops the greater part of the two principal spines ; its structure and 

 contents are the same as in the other Acanthonida. 



Synopsis of the Genera of Aniphilonchida. 



( Two opposite large principal spines (frontal and 



Eighteen .smaller spines of nearly ; caudal) equal, .... 341. AmiMlonche. 



equal size and similar form. '\ 



( Caudal spine larger than the frontal spine, . 342. Amphibelone. 



Eight tropical and eight polar spines of different sizes (tho latter often rudimentary), . 343. Acantholonche. 



