In that species of _Murex_,[110] which, on account of the long and slender rostellum, and the spines with which it is covered, is known to collectors as the Thorny Woodcock (_M. tenuispina_), the shelly spines of the earlier whorls would interfere with such as came, in process of development, to be superposed on them; for they cross the area which is to be the cavity enclosed by the advancing lip. They are, however, removed by absorption; but not so completely but that traces may still be discovered where they formerly existed: evidences of the quondam existence of what exists no longer. Towards one side of the upper surface of the pretty Star-fish, _Cribella rosea_, (as in many other species of Star-fishes,) there is a curious little mark, known as the _madreporic plate_, the use of which has greatly puzzled naturalists. Sars, the Norwegian zoologist, has unveiled the mystery.[111] The young larva, before it assumes the stellar form, is furnished with a sort of thick column, divided into four diverging clubbed arms, which are adhering organs, ancillary to locomotion. In the process of development, however, new locomotive organs are formed; and this four-fold column, being no longer needed, sloughs away; and that so completely, that not a trace of its existence remains, _except this scar_, or "_madreporic plate_;" which is therefore a permanent record of something that has quite passed away. But the closest parallel to the relation borne by the skeleton of an extinct species to an extant one, is presented by that of the hilum to a seed, or of the umbilicus to a mammal. Each of these is a legible and undeniable, record of a being, whose individuality was totally distinct from that of the being by which it is presented, and of which all vestiges have disappeared, _save this record_. Nor is the parallel founded on obscure or rare examples; both the umbilicus and the hilum are generally conspicuous; and both are extensively found in their respective kingdoms, the former pervading the viviparous Vertebrata, the latter characterising the whole of the cotyledonous types of vegetation.