GUAl'TOLITES OF NEW YOUK. I'AUT ] 



485 



BecUned series 



Reflexed 



series 



Declined series 

 Dependent series 



Fig. 1 Diagrammatic figure, showingr tlie relatire 

 angles of divergence nf tlie ijrunclies characteristic of 

 tiie several series of Didymograpti [Copied from 

 Lapwortb, Eilea & Wood] 



conception of the angle of divergence of the branches. The fact of the 

 gradual leversion of the branches from a descending to an ascending position 

 is readily expressed by the use of Hopkinson's definition of the angle of 

 divergence as the "angle included within the polypiferous margins of the 

 branches." 



The variation of the angle of divergence and of the curves described by 

 the branches, has made it expedient 

 to introduce terms for a number of 

 characteristic positions of the branches, 

 which will save long descriptions. 

 These are dependent, declined, dejlexed, 

 horizontal, reflexed, reclined and recum- 

 bent hranches. The terms find their 

 explanation in the appended diagram. 



A difference has been noted be- 

 tween dichotomous and monopodial 

 [Wiraan] or lateral [Elles] branching. In the first both diverging mother 

 thecae of the new branches assume directions different from that of the 

 older branch, while in the monopodial branching only one mother theca of a 

 new branch turns aside. Where repeated divisions produce numerous genera- 

 tions of branches, the self-explaining terms, brandies of the first, second, etc. 

 order, are used to denominate the successive generations. 



The receptacles of the zooids passed originally under the designations, 

 denticles, calycles, cellules, cells, cttps and Zdhne, and were later on called 

 hydrothecae like the analogous parts of the hydrozoans. At present the term 

 thecae, {Thehen, Ihehorna) has found nearly universal usage. Lapworth has 

 lately used the term, graptothecae [1897, p.251]. Frequently the zooids, which 

 once inhabited the thecae, are meant by the expression thecae, a usage which, 

 though illogical, can, as Holm has pointed out, hardly be avoided, if long 

 paraphrases are not to be used. As a matter of fact, the shells are in paleon- 

 tologic literature currently treated as the individuals. The thecae represent 

 the persons of the first order. 



