OCR Text |
Show (J58 MR. F. J. BELL ON THE ECHINOIDEA. [June 1/, the eastern coast of America, was first described by Alex. Agassiz, is distinguished by its form, its small anal system, and the presence of large plates on the buccal membrane, at the point where this structure unites with the test. In bis definition of the form \ Agassiz states that " the anal system and the actinostome are comparatively smaller in the West Indian species." In support of this statement he gives, however, onlv one set of measurements for T. depressus; but they hardlv bear out bis proposition, inasmuch as in the specimen described by him, which had a long diameter of 127 mm., the anal system measured 9 m m . ("riving a percentage value of 7*08), whereas the four values to be rained from his measurement of H. esculenta are respectively 15, 9*2, 8*1, and 7*9. The single specimen of T. depressus in the possession of the British Museum gives a percentage value of 6*6 (the anal system measuring 8 m m . and the long diameter 120 mm.). We mi°ht, indeed,imagine that a "than" had dropped out in the sentence just quoted, were it not that it does as it stands state fairly enough the comparative relations presented by the actinostome in the two forms therein mentioned. To this structure we will now turn. In Prof. Agassiz's specimens the actinal system measured 29 ram. (percentage value 22*8) in H. depressa, and 26*2 m m . (percentage value 22*2) in the largest specimen of H. esculenta of which he gives the measurements. The differences here are indeed not very great, but, such as they are, are evidence against Prof. Agassiz. As, however, m y remarks are based rather on what I have been able to observe in the specimens in the national collection than on deductions from Prof. Agassiz's measurements, I am able to give in m v adhesion to the statement already quoted, that the actinostome is smaller in the West-Indian species; for I find that while the British Museum specimen of T. depressus gives a percentage value of 25 for the actinostome, that of T. esculentus does not exceed 23*2, and mav fall as low as 18*8 per cent. T. esculentus and T. angutosus.-The diagnoses of Prof. Alex. Agassiz are notoriously difficult; but, so far as an attentive study of his remarks on these two species are of value, they appear to me to be convertible into tbe following propositions:- The species T. angulosus is distinguished from the West-Indian T. esculentus by the following points: (i.) The tubercles are smaller in size and less in number. (ii.) The anal system is comparatively very large. (iii.) The abactinal system is more circular and less pentagonal, owing to the smaller size of the genital plates. (iv.) The poriferous zone is much narrower. (v.) The actinostome is larger. (vi.) The spines are much more slender. (vii.) The anal plates are smaller and more numerous. The descriptions are marred by a very remarkable misprint, which states in effect that the specimens of angulosus measured by Prof. Agassiz have a height nearly twice as great as their long 1 Revision of the Echini, p. 500. |