OCR Text |
Show 438 PROF. F. J. BELL ON THE TEMNOPLEURIDAE. [June 1, Absolute diameter in millims. 14 21-5 28 34 36 Height. 92-8 84-1 92-8 911 102 Percentage value of Abactinal area. 21-4 18-6 17-8 17*6 169 Anal area. 10 9-3 8-8 10 Actinostome. 35-7 30-2 28 24 27*7 Poriferous zone. •9 11 •2 The diminution in the proportional size of the abactinal or actinal areas is very fairly shown by this Table. The auricular foramen is sometimes almost a complete square ; the auricles vary a little in height, but the connecting ridges are always low. The spines may be all white, or they may be of a light brick-red. 4. AMBLYPNEUSTES PALLIDUS. Just as it is possible to distinguish a number of forms which, by their exquisite pattern, are seen to be specimens oi A. formosus, so, too, it is possible to separate off a series of forms in which the pattern on the test is only visible on very close examination ; these can at any rate be ranged under the head of A. pallidus. When the succeeding Table of measurements is compared with that of A. formosus, it will not be hard to formulate certain differences between them: the fifth specimen is interesting as presenting the oviform arrangement in having the height greater than the diameter; and here, as in A. formosus, we find an example of how greatly the species of this genus may vary in form. W e find, moreover, that in A. pallidus (saving always specimen v.) the abactinal area increases in size with an increase in diameter, whereas in A. formosus it diminishes very markedly; so, again (again excepting v.), we find the actinostome to be not much larger or much smaller proportionately in specimens of different sizes, while in A. formosus the actinostome diminishes very much in size at a comparatively early stage, and then varies within considerable limits. These, however, though real characters, are not easily grasped, and it is difficult to say what points of distinction can be at once appealed to as readily accessible to the zoologist. Prof. Alex. Agassiz states that the only points of difference which he can find between them are : - (1) The slight development in A. pallidus of the coloration and pattern of sculpture so distinctive oi A. formosus. (2) The presence of secondaries and small miliaries in the narrow poriferous zone of A. pallidus. (3) The less numerous median sutural pores of the same form. (4) The greater uniformity [in size] and more irregular arrangement of the tubercles of A. pallidus1. 1 I base this view on the supposition that the word " separate," in line 9 of p. 482, is a lapsusplntneB for "unite." |