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



primary aspinal pores not different from the others. Spines very thin and long, cylindrical ; four to 

 six times longer in the outer than in the inner part. 



Dimensions. Length of the shell 0'24, breadth 0'16 ; breadth of the spines - 002. 



Habitat. Mediterranean (Messina), surface. 



Family XLIII. HEXALASPIDA, n. fam. (PL 139). 



Definition. ACANTHARIA with a simple discoidal or lenticular lattice-shell, com- 

 posed of the branched apophyses of twenty radial spines meeting in the centre 

 and disposed according to the Mullerian law of Icosacantha. Six larger spines in the 

 hydrotomical plane, prominent on the margin of the circular or elliptical biconvex lens. 

 Fourteen other spines much smaller or rudimentary. Central capsule biconvex lenticular, 

 enclosed in the fenestrated shell. 



The family Hexalaspida represents a new small, but very interesting group of 

 Acanthophracta, which differs from all others in the lentelliptical or triaxial form of 

 the lenticular lattice-shell, the margin of which bears six larger spines placed in the 

 hydrotomical plane (compare above, p. 719). They may therefore be characterised shortly 

 as " Acanthophracta lentelliptica," with three different dimensive axes and six larger 

 marginal spines. A closer comparison with the other ACANTHARIA leaves no doubt 

 that the Hexalaspida must be derived from the Belonaspida by stronger development of 

 six radial spines placed in the hydrotomical plane, namely, two equatorial and four 

 associated polar spines ; whilst the six spines of the geotomical plane (perpendicular to 

 the former) are much smaller ; the eight tropical spines are intermediate in size between 

 the former and the latter. 



The geometrical fundamental form of the Hexalaspida (of the central capsule as well 

 as of the enclosing shell) is therefore lentelliptical, with three different dimensive axes, 

 and they exhibit among the ACANTHARIA a relation to the spherical Dorataspida 

 and the ellipsoidal Belonaspida similar to that which the lentelliptical Larcoidea 

 exhibit to the spherical Sphseroidea and the ellipsoidal Prunoidea among the 

 Sphserellaria (compare above, p. 599). The largest of the three dimensive axes 

 (which are perpendicular to one another) is here the hydrotomical axis, the shortest, on 

 the contrary, the geotomical axis ; the intermediate in size being the spineless axis. 

 The development of the whole body is strongest in the hydrotomical meridian plane, 

 in which the six principal spines are placed ; it is weakest in the geotomical plane, in 

 which the six smallest spines are placed ; the eight tropical spines are intermediate in 

 size between the others. This peculiar development is illustrated by the figures of 

 PI. 139, where the four equatorial spines are everywhere marked by c, the eight 

 tropical spines by b and d, the eight polar spines by a and e. 



