The form and relative position of the parts above described, afford characters by which the order is divided into genera. The mouth, which is situated on the lower part, is in some species furnished with five sharp angular teeth, attached to a calcareous framework that admits of being protruded; this apparatus, when entire, commonly bears the name of "_Diogenes' lantern_." The eminences on the surface of the plates vary in size from mere granules, or papillæ, to large mamillated tubercles; they serve for the attachment of movable spines, which also present great variety of form and ornament. The spines have a cup-like cavity at the base which fits on the papillæ, and in many species are only supported by the capsular envelopment of the common integument; but in others, the large spines are attached by a ligament which passes from the centre of the socket, and is received in a perforation of the papilla of each tubercle, in the same manner as the _ligamentum teres_ of the human thigh-bone. Transverse slices of the spines exhibit the internal structure, and are beautiful objects under the microscope. There are also minute appendages to the integument, called _pedicellariæ_, or pinchers, of a very remarkable character, whose functions are not known. They are slender columnar bodies, each crowned with three calcareous teeth-like spines, beautifully sculptured, and which in some species are long and slender, in others short and obtuse. I have not observed any traces of these bodies, even in the best preserved echinites, but as they are as durable as the spines and case, they may exist in a fossil state. [Sidenote: TURBAN ECHINITES.] This general view of the structure of the recent Echinites will enable the student to understand the nature of the fossil remains; for the parts above described, are found more or less perfectly preserved, either in their natural arrangement, or separated and dispersed in the rock. The habits of these animals, of burrowing in the sand, were favourable to their preservation in a mineralized state; and in some of the oolitic limestones, hundreds of beautiful examples of Turban Echinites (_Cidares_), having their spines spread out on the face of the rock, are found lying in the positions they evidently occupied when living. The quarries near Calne and Chippenham, in Wilts, are celebrated for such fossils. It was my intention to give figures of the genera into which the numerous fossil species have been divided by modern authors: but I found the attempt hopeless, from the changes in nomenclature and arrangement which are constantly taking place. The monographs on the fossil Echinidæ, by Prof. E. Forbes, now in course of publication by the Government School of Mines, will, when completed, place this branch of Palæontology on a satisfactory basis.