The variation in the number and direction of the spines is a striking feature in Spondylus. When the whole lower surface adheres to branches of coral, a very frequent occurrence, they are confined to the upper valve, but when a part only of the valve, the whole surface becomes covered. Having finished our short sketch of the Conchifera, we shall now treat of the singular group, Brachiopoda,[11] which some place nearer to the Gasteropoda than the Pteropoda, giving them, in fact, their place. It is out of the province of this work to enter into the physiological arguments of such a question. The days of the Brachiopoda or short-footed animals are past. Of the 1842[12] species formerly known, a few types of a small number of genera only are left, numbering in all 102. The Terebratulidæ are best represented; there were once 300 or 400 species; there are now not more than 67 in the seas of the world. The difference between the past and the present is especially striking, when we compare the recent and fossil species of Europe. Among no other class of shells has there been such a wholesale extinction of species. The great family of Spiriferæ are wholly extinct, and of 400 Rhynconella only four are now living. The curious Crania, Discina, and Lingula are still living, and are mostly found in the seas of the southern hemisphere. FOOTNOTES: [Footnote 9: We give this illustration as representing the comparative size of the oysters at different ages; but it is necessary to state that the specimens were artificially attached to the block by means of glue for exhibition. Oysters always attach themselves by the back of the rounded shell near to the hinge, as stated at p. 363.] [Footnote 10: The cause of the present scarcity of oysters is a much-vexed question. Mr. Frank Buckland, the greatest living authority on oyster and fish culture, attributes it to sudden changes of temperature at the critical period when the spat is newly formed, rather than to over-dredging.--ED.] [Footnote 11: The _Brachiopoda_ may be thus tabulated:-- Family. I. _Lingulidæ_, containing Lingula and other fossil genera. II. _Discinidæ_, containing Siphonolreta and Discina. III. _Craniadæ_, containing Crania. IV. _Productidæ_, containing Chonetes and Productus, fossil. V. _Orthidæ_, containing Calceola, Davidsonia, Strophomena, and Orthis. VI. _Rhynconellidæ_, containing Atrypa, Pentamerus, and Rhynconella. VII. _Spiriferidæ_, containing Uncites, Retzia, Athyris, and Spirifera. VIII. _Terebratulidæ_, containing Thecidium, Agriope, Terebratella, and Terebratula. ] [Footnote 12: Woodward's Manual, p. 135.] CEPHALOUS MOLLUSCA. CHAPTER XIII. GASTEROPODA.