W. D. Lang — On Stomatopora antiqua. 259 



It is to facilitate the comprehension of the method of investigation 

 employed that the reader is referred to the paper on " The Jurassic 

 Forms of Stomatopora and Proboscina."^ In this paper great prominence 

 was given to the mode of branching ; and it was pointed out that 

 the remarkable regularity of this character was peculiar to the 

 Jurassic forms. Three types of branching were described as Type I, 

 the Intermediate type, and Type II ; and the order of succession was 

 invariable, the first passing gradually into the second, and the second 

 into the third type. If it may be inferred from the irregularity 

 of the branching in the few Silurian specimens of Stomatopora 

 examined that the pre-Jurassic ancestors of the species under 

 consideration were irregular in their mode of branching, the Jurassic 

 regularity appears in the Lower Lias form as a new character 

 superimposed upon a pre-Jurassic irregularity. The apparent 

 expression of this is that although in the Lower Lias form the 

 regular dichotomons branching after Type I, the Intermediate type, 

 and Type II may be observed, it is somewhat obscured by a slight 

 irregularity, and the daughter zooecia at each dichotomy are of 

 unequal length. Thus a second character becomes involved — the 

 proportion of the length of the zooecium to the breadth. These 

 points are illustrated in PI. XIV, Figs. 4 and 5. 



Moreover, a slight irregularity occurs in a third character — the 

 shape of the zocecium. In the Oolitic forms primitive cylindrical 

 zooecia become pyriform, and in more specialized forms often revert 

 to a cylindrical shape. In Stomatopora antiqua, Haime, though the 

 general sequence is cylindrical — very slightly pyriform — cylindrical 

 by far the greater part of the zoarium having cylindrical zooecia, 

 occasionally a comparatively pyriform zooecium occurs without any 

 relation to the general sequence in shape. This is well shown in 

 Terquem & Piette's figure of S. antiqua, Haime,'^ and in the 

 specimen B.M. D. 7623 (PI. XIV, Fig. 5). It is true that such 

 sporadic irregularit}"^ in any one character is liable to occur to some 

 extent among the most regular Jurassic forms. And the explanation 

 is suggested that so plastic and undifferentiated are these primitive 

 forms that rigid stability of character is not to be expected in 

 the same degree as in the case of the ornamentation of such a highly 

 specialized group as the Ammonites. 



Mr. Buckman has offered another explanation of the sporadic 

 appearance of the comparatively pyriform zooecia. Jackson' has 

 shown that in plants a new branch tends to recapitulate former 

 characters more primitive than those peculiar to that part of the 

 plant whence it springs. He applies this to animals as well as 

 to plants, and voices -it in the law that " throughout the life of the 

 individual, stages may be found in localised parts, which are similar 

 to stages found in the young, and the equivalents of which are to be 

 sought in the adults of ancestral groups." 



^ "W. D. Lang: loc. cit. 



- 0. Terquem et E. Piette, " Le Lias inferieur de I'est da la France " : Mem. Soc. 

 Oeol. France, ser. ii, vol. viii (1868), p. 124, pi. xiv, fig. 32. 



' E. T. Jackson, "Localised Stages in Development": Mem. Boston Soc. Nat 

 Hist., vol. V, Xo. 4 (1899), p. 92 ; see also pp. 139, 141. 



