The heart was tubular in form, 3a and **3b**, Two crustacean teeth extending the whole length of the (of Dithyrocaris) from the body, and having a pair of ostia Carboniferous series of in each somite.

X is, in general, a determinate point, the barycentre of aA, **3B**, &c. (or of A, B, &c. for the weights a, 0, &c.).

2-}-cos B It is readily shown that the latter gives the best approximation to 0; but, while the former requires for its application a knowledge of the trigonometrical ratios of only one angle (in other words, the ratios of the sides of only one right-angled triangle), the latter requires the same for two angles, 0 and **3B**.

The sum of two weighted points aA, (**3B** is, by definition, the point (a+13) G, where G divides AB so that AG: GB =0: a.

+X) happens to be zero, X lies at infinity in a determinate direction; unless - aA is the barycentre of (**3B**, yC, ...

Even when seen in minute features only he recognized them as constant progressive characters or " chronologic varieties " in **3b** --i C D E F G H I -14-21 -I-31 1 - I - 41 contrast with contemporaneous or " geographic varieties," which he considered inconstant and of slight systematic value.

Sc) = (Sa-23aSb +**3b**) (Oa - Se) =Sz - 23QSb+303 b - SQS c +23a3 b 3c - StSc = 6a 3 - 4a2b1 +2a,b 2 - 2a2c1 +2alblci - 2b2c1 =2 (al - 3a1a2+3a3) = 2 (3) .