Without attempting to answer this question categorically, it may be pointed out that within the limits of the family (Ptychoderidae) which is especially characterized by their presence there are some species in Y art dY **YY** cts, posterior limit of collar.

In the general motion again of the liquid filling a case, when a = b, 52 3 may be replaced by zero, and the equations, hydrodynamical and dynamical, reduce to d =- 2+ 2 J, = 2 x22111, d = 2 2`2 (+/'15-Om) (1 **yy** y n`t dt a +c dt a +c dt a +c) dc2, a2-1-c2 d122 a2 c2 dt ="2) +a2= G2y 71' dt = 121 1 - a 2 -c 2SJ, (19) of which three integrals are e +777 r z y 2= L -?2J2, (20) (a2 + c2) 2 2 121+14 =M+ 2c2(a2-c2)1 ' (21) 121+522hN = + x24 2,2 and then (dt / 2 = (a2 + c 2) 2(° v 2 - 12171) 2 4C4 2 2 - (+ c2)2?(E+77) (?

Kirchhoff's expressions for X, Y, Z, the coordinates of the centre of the body, FX=y 1 cos xY--y 2 cos **yY**-{-y 3 cos zY, (18) FY = -y l cos xX -Hy2 cos yX+y 3 cos zX, (Ig) G=y 1 cos xZ+y 2 cos yZ+y 3 cos zZ, (20) (21) F(X+Yi) = Fy3-Gx3+i /) X 3epi.

But, assuming the distributive principle, the product of two lines appeared to give the expression xx' - **yy**' - zz' +i(yx' +xy')+j(xz' i j (yz' +zy').

He had now the following expression for the product of any two directed lines: xx' - **yy** - zz' +i(yx'+ xy')+ j(xz' '+zx') +ij(yz' - zy').

And now a directed line in space came to be represented as ix+jy+kz, while the product of two lines is the quaternion - + **yy** ' +2z') +i (yz ' - zy') +j (zx' - xz') +k (x y ' - yx').

Bay ll c t ' LY Q " **yy** be Pony.