238 



KNOWLEDGE 



TDecembeb 1, 1891. 



con-esponding to the one directed upwards in Fig. 1. 

 Here, tberefore, ■we Lave the bilateral symmetry still more 

 distinctly marked. Another advance shown in this Urchin, 

 is the circumstance that the perforated portions of the 

 ambulacral areas do not extend on to the lower sui-face of 

 the test, but stop short at the edge. Some members of 

 the same family are still more peculiai' in that the apical 

 disc is greatly elongated from front to back, in consequence 

 of which the five ambulacral areas do not all meet one 

 another at the summit of the test ; those conesponding to 

 the three turned away fi'om the spectator in Fig. 1 meet 

 ing near the middle of the test, whDe the remaining two 

 are brought together in the opposite pait of the test 

 corresponding to the lower portion of Fig. 1. 



A third common chalk species, the Heart-Urchin, or 

 Fauy Heart of the quanymen {^ilicrnster), aflbrds a good 

 example of what may be regarded as the extreme modifi- 

 cation of structure developed in the group. The upper 

 surface of one of these Urchins is represented in Fig. 4, 

 from which it will be seen that the contour- is regularly 

 heart-shaped, and the whole test much depressed. The 

 perforated portions of the ambulacral areas are now 

 restricted merely to the central region of the upper sur- 

 face, one of these areas (directed upwards in the figure! 

 forming a shallower groove, and being otherwise markedly 

 different fi'om all the others. In such an Urchin the 

 bilateral symmetry is very strongly marked indeed, and, 

 since the upper border of the figure represents the 

 anterior, and the lower the posterior extremity of the 

 animal, we may compare the three anterior ambulacral 

 areas to the head and arms of a quadruped whose hind 

 legs will be represented by the two posterior ambulacra. 

 In regard to the position of the two orifices of the test in 

 this species, the vent is situated on the flattened posterior 

 surface, near its junction with the upper surface, while the 

 mouth occupies a position nearly midway between the 

 centre and the anterior border of the lower stirface, at the 

 commencement of the groove formed by the anterior 

 ambulacral area. The mouth is peculiar in that it does 

 not open directly on the surface, in the usual manner, but 

 has a projecting hp by means of which its aperture 

 assumes a forward direction. The common Purple Heart- 

 Urchin I SjidUnigus) of our present seas is a larger repre- 

 sentation of this group, presenting the same general type 

 of structure. 



We thus see how gradual is the passage from a species 

 of the type of the Common Urchin to that of the Heart- 

 Urchin, widely different as are these two from one another. 

 ^Ye might, indeed, proceed further with our investigations, 

 and show how certain of the Irregular Urchins have become 

 so flattened as to assume the form of thin plates, which 

 in some instances are deeply notched at their periphery. 

 And we might also investigate the variation of form and size 

 displayed by the spines of the ditferent gi'oups. Enough 

 has, however, been written for our present object, which 

 has been to show the amount of interest that attaches to 

 the investigation of the lines of modification on which 

 development has proceeded among the Sea-Urchins. This 

 has shown how a regular progressive advance in one 

 particular direction has taken place fi-om the earlier to the 

 later members of the group; and we thus have another 

 excellent instance testifying in favour of the doctiine of 

 evolution. This brief sketch may possibly give additional 

 pleasure to a sea-side sojourn, by inducing some of oiu- 

 readers to direct their attention, first of all, to the recent 

 Sea-Urchins, after which they will scarcely fail to extend 

 their investigations to the fossil species so abundantlv 

 distributed through our rocks; and, we will venture to add, 

 that if they do so theii- interest cannot fail to be aroused. 



THE FACE OF THE SKY FOR DECEMBER. 



By Herbebt Sadleb, F.R.A.S. 



THE markings on the solar surface show no signs of 

 diminution. The following are conveniently ob- 

 servable minima of some Algol-type variables (</. 

 " Face of the Sky '' for November). U Cephei. — 

 December 1st, lOh. 9m. p.m. ; December 6th, 9h. 

 49m. P.M. ; December 11th, 9h. 28m. p.m. ; December 16th, 

 9h. 8m. P.M. ; December 21st, 8h. 48m. p.m. ; December 

 26th, 8h. 28m. p.m. ; December 31st, 8h. 7m. p.m. Algol. — 

 December loth, llh. 43m. p.m. ; December 18th, &h. 31m. 

 P.M. ; December 21st, oh. 20m. p.m. A minimum of the 

 variable star ilira ( Omicrou I Ceti will occur on the 29th 

 of December. The magnitude at minimum varies from 

 8 to 9i magnitude, and the star is of a fiery red colour at 

 that stage. 



Mercury is an evening star, and would be well situated 

 for observation during the first three weeks of the month 

 but for its great southern declination. He sets on the 1st 

 at 4h. 42m. p.m., or 49m. after the Sun, with an apparent 

 diameter of oi", and a southern declination of 25'' 49', 

 f|j of the disc being illtmiinated. On the 18th he sets at 

 oh. Bm. P.M., or Ih. 19m. atier the Sun, with an apparent 

 diameter of 7'0", and a southern declination of 24^ 36', 

 3%% of the disc being Uluminated. About this time he is 

 at his brightest. On the 18th he sets at oh. 10m. p.m., or 

 Ih. 20m. after the Sun, with an apparent diameter of 80", 

 and a southern declination of 23° 20', -[%% of the disc 

 being illuminated. After this he rapidly approaches the 

 Sun, coming into inferior conjunction with him on the 

 28th. He is at his greatest eastern elongation (20^°) on 

 the 11th, and in conjunction with X Sagittarii on the 

 morning of the 7th, but the phenomenon will not be visible 

 in Europe. While visible he describes a direct path in 

 Sagittarius. Venus is also an evening star, but is almost 

 as indiflerently placed for the observer as Mercury is. 

 She sets on the 1st at 4h. o6m. p.m., or Ih. 3m. after the 

 Sun, with a southern declination of 24" 23', and an ap- 

 parent diameter of 10|', ^'o of her disc being illuminated, 

 while her brightness is less than one quarter of what it 

 was at the beginning of the year. On the 31st she sets at 

 6h. 4m. P.M., or 2h. 6m. after the Sun, with a southern 

 dechnation of 20° 3-5', and an apparent diameter of 1\^", 

 -^ of the disc being iDuminated. During the month she 

 passes from Sagittarius into Capricoruus, but without ap- 

 proaching any conspicuous star. 



Both Mars and Uranus are, for the puqjoses of the 

 amatem-, invisible ; and Saturn does not rise on the last 

 day of the month till llh. 9m. p.m. We therefore defer our 

 ephemeris of him till next year. .lupiter is still favourably 

 placed for observation as an evening star, but should be 

 observed as soon as possible after sunset. He sets on the 

 1st at llh. 33m. p.m. with a southern declination of 9° 12', 

 and an apparent equatorial diameter of 40| ', the phasis 

 on the preceding hmb amounting to 40^". On the 31st he 

 sets at 9h. 46m" p.m. with a southern declination of 7^ 37', 

 and an apparent equatorial diameter of 36f". The fol- 

 lowing phenomena of the satellites occur while .Jupiter is 

 more than 8° above, and the Sun 8° below, the horizon. 

 On the 1st an occultation disappearance of the first satellite 

 at 6h. 12m. p.m., and an eclipse reappearance of the same 

 satellite at 9h. 49m. 21s. p.m. On the 2nd a transit egress 

 of the first satellite at 5h. 39m. p.m., and of its shadow at 

 7h. Om. P.M. On the 4th an occultation disappearance 

 of the second satellite at 7h. 7m. p.m., and an echpse 

 disappearance of the fourth satellite at 9h. 25m. lis. 

 On the 5th a transit egress of the third sateUite at 



