Page:Scientific results HMS Challenger vol 18 part 1.djvu/1080

872 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 0.002.

Habitat.—Mediterranean (Messina), surface.

Definition.— with a simple discoidal or lenticular lattice-shell, composed of the branched apophyses of twenty radial spines meeting in the centre and disposed according to the Müllerian 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 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 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 a relation to the spherical Dorataspida and the ellipsoidal Belonaspida similar to that which the lentelliptical  exhibit to the spherical  and the ellipsoidal  among the  (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 Pl. 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.