Fig. 249.—Pourtalesia jeffreysi, slightly enlarged. (From Wyville Thomson.)
Fam. 5. Palaeostomatidae.—An aberrant family consisting of one genus, Palaeostoma. Petals grooved, with a peripetalous fasciole, but peristome central and pentagonal.
Fig. 250.—Hemiaster philippi. × 2. (From Wyville Thomson.)
Fam. 6. Spatangidae.—Spatangoidea of more or less flattened shape, with well-marked petals and a sub-anal plastron as well as the ventral one. One fascicle at least, but a peripetalous one never present. The anterior ambulacrum grooved and different from the rest. This family is represented in British waters by two genera, Spatangus and Echinocardium. The former possesses only a sub-anal fasciole, and has specially long curved spines on the ventral plastron. It is represented by two species, S. purpureus and S. raschi, the latter being distinguished by a pointed lower lip. It is a deep-water species, found in 100 fathoms and over on the west coast. S. purpureus is fairly common in rather shallow water. From observations made on specimens kept in confinement it appears to burrow only so far as to leave the petals uncovered; hence there is no need of a peripetalous fasciole. Echinocardium is devoid of the thicker spines on the plastron, and has an internal fasciole and a perianal one as well as the sub-anal. As already mentioned, it is a deep burrower. It is represented by three species, E. cordatum, E. pennatifidum, and E. flavescens. The first, described as the type of the Spatangoidea, has a deeply grooved anterior ambulacrum. In the remaining two species this ambulacrum is not grooved. E. flavescens has only six or seven pairs of pores in the posterior petals, E. pennatifidum twelve to fourteen. Both come from deeper water than E. cordatum.
Fam. 7. Brissidae.—Allied to the Spatangidae, but distinguished by sunken petals and a peripetalous fasciole.
Two genera are recorded from the British area, Schizaster and Brissopsis, but the first has only been found once in deep water; the second is common. Schizaster has the front petals three times as long as the hind ones, and no sub-anal fasciole. Brissopsis has the front and hind petals of about the same length, and a sub-anal fasciole. The only British species is called B. lyrifera, on account of the fiddle-shaped outline of the peripetalous fasciole.
Hemiaster (Fig. 250) in general resembles Schizaster, but the petals are equal in length, and the two posterior serve as brood-pouches for the young. This genus is mainly Antarctic.
