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Show 1879.] MR. F, J. BELL ON THE ECHINOIDEA. 255 final boundary of the ambitus, and is never closed ; in M. grandis it forms a slightly convex line, which bounds the posterior end of the actinal plastron, and then turns upwards at a very open angle; in M. ventricosa the horizontal line is straighter, the lateral bands longer and almost perpendicular to the former. In Brissus the same fasciole is a little broader, a m d is always closed, the resulting figure being cordiform, often more or less truncated at its base. In Metalia the fasciole around the subanal plastron is still broader ; and there is, in addition, a narrower band on either side of the anus, which extends just beyond the ambitus, and is of the form of the fasciole in Meoma ventricosa : the result is, that we have the effect of the presence of the fascioles of both Meoma and Brissus in Metalia. Whatever value these characters have from a genetic point of view, there can be no doubt as to their ready accessibility and general constancy. To resume. Meoma displays the simplest form of subanal fasciole, and the most general distribution of the primary tubercles ; the anterior ambulacrum is but slightly depressed; and the anterior lateral ambulacra are to the posterior ones in the proportion of from 1000 . 1000. 1081 l 0 1135* In Brissus the subanal fasciole is closed, and the larger tubercles are absent from the posterior portion of the abactinal surface ; it is rarely that the anterior ambulacrum is depressed beyond the level of the test; and the lateral ambulacra stand to one another in the proportion of from ^ to ^~ (in adult specimens the ratio barely exceeds iyoo). The subanal plastron is provided with three or four pores on either side, but there are no radiating bands. It is in Metalia only that the anterior ambulacrum is ever found in a deep and well-marked groove; the larger tubercles are confined within the peripetalous fasciole ; the subanal fasciole gives off bands to either side of the anus; and the lateral ambulacra are to one another in ratios varying from 1950 to j ^ 1 . The subanal plastron may have as many as nine pores on either side; and well-marked radiating lines extend outwards to them from the more median region of the plastron. 1 Species. (ii.) „ », _ ("•) Length of ant. amb. 49, 49 52, 52 72, 72 69, 69 65, 65 68, 68 Length of post. amb. 53, 53 59, 59 90, 90 87, 87 90, 90 98, 98 Proportion. 1000 to 1081 1000 to 1134 1000 to 1252 1000 to 1260 1000 to 1384 1000 to 1441 |