PART I. 



COMPARATIVE MORPHOLOGY OF ECHINI. 



Having enjoyed the opportunity of studying most of the genera and species of Palaeozoic 

 Echini, it is of importance to sum up the comparative structure and morphology of these types 

 and to show their relations to living forms. 



Palaeozoology is only one aspect of zoology, distinguished from the study of living animals 

 about as embryology, a study of the young, is in so far separated from a study of adult organ- 

 isms. As Huxley said, the only difference between a collection of fossils and a collection of 

 recent animals is that one has been dead longer than the other. For an intelligent study of 

 fossils their living representatives must be constantly borne in mind, as their details of struc- 

 ture throw a flood of light on fossil forms. Conversely a study of fossils in relation to the living 

 gives a knowledge of the earlier and often primitive representatives of a group, throws light 

 on the structure of the adult, and especially of the young of living types, and gives a rounding 

 out of the knowledge of a group that cannot be attained by a si-udy of the living forms alone. 

 This is my excuse, if such is necessary, for including in this discussion representatives of living 

 and other post-Palaeozoic Echini in the summing up of the relations and structure of the Pal- 

 aeozoic types, with which this memoir is primarily concerned. 



Under the consideration of Morphology are taken in order the form of the test, orienta- 

 tion, the pentamerous system and variation, the structure of the skeleton, and growth. In 

 the sea-urchin there are many anatomical parts for consideration, and they are taken up in 

 the following order: the ambulacrum and interambulacrum of the corona, basicoronal plates, 

 imbrication, spines, peristome, ocular and genital plates, periproct, Aristotle's lantern, and 

 perignathic girdle. 



Form of the Test. 



The general shape of the test of a sea-urchin varies greatly, but is more constant in Palaeo- 

 zoic than in post-Palaeozoic types. The test may be elliptical with the vertical axis somewhat 

 longer than the horizontal through the mid-zone, Bothriocidaris, Palaeechinus ellipticus (Plate 

 29, fig. 2), Lepidesthes colletti (Plate 69, figs. 2-6). In Recent Echini the test is occasionally 

 elliptical, as in Amblypneustes formosus Valentin, a species in which the height may exceed 

 the diameter, though not always (A. Agassiz, 1873, p. 479). The test may be nearly or 



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