.3^. OBSERVATIONS ON THE BRACHIOPODA. 607 



Neoembryo ; the true larval stage as Typembryo, with the addition 

 of Dr. R. T. Jackson's term Phylembryo for the later phases of the 

 last-named stage, when the animal, for the first time, can be referred 

 ■definitely to the class to which it belongs. 



It is at the close of the phylembryonic stage that the palaeontologist 

 j)ey se should commence his investigations, for the initial shell, the 

 protegulum, is then developed. There is a remarkable similarity 

 in its appearance in all forms. It is noteworthy that " representatives 

 ■of such distinct groups as Rhyuchonella, Spirifer, Athyris, Nucleospira, 

 and the Meristoids resemble each other so closely at this stage that the 

 genus can be then determined only by comparatively trivial features " 

 (7). Nearly twenty years ago, Morse pointed out the Linguloid 

 form of the shell assumed in " the early stages of Terebraiulina." It 

 now appears that thq, adolescent Shizocrania resembles the youthful age 

 of Orbicnloidea, and that the young Orhiculoidea recalls the genus Paterina 

 in the " paterina stage " of its development. The protegulum has 

 been observed by Mr. Beecher in forty genera of the leading members 

 of the class, recent and fossil (8). It is described as always corneous 

 and imperforate. The genera with pedicles sufficiently long to admit 

 of free axial movement have elongated and rostrate shells (8). 

 Breadth of form is the correlative of a short pedicle, while the 

 •discinoid resulted from mechanical conditions of growth. 



The brachiopod next passes through its youthful or " nepionic " 

 •stage, when the shell is smooth, or with few plications, into the 

 adolescent or " nealogic " period, and attains, at maturity, the 

 *' ephebolic " condition — the maximum of individual or ontogenetic 

 development. Varieties and abnormalities usually begin to express 

 themselves early in the development of the shell, and we learn that 

 " such divergence from the normal form rapidly increases as maturity 

 approaches (7), and they are gradually diverted from the harmony 

 characterising the incipient stages of other members of the group," 

 Soon after becoming adult, they commence to age, and pass through 

 first a "clinologic" stage of arrested growth, and, subsequently, the 

 •*' nostologic " phase of absolute senility. 



These latter stages are well represented in the well-known, widely- 

 distributed lobate Silurian orthoid O. biloba, Linne, by the disappear- 

 ance of the bilobed condition in old age, as Mr. Beecher has fully 

 described in his paper on the " Development of Bilobites " (10). In 

 the degenerate form from the Lower Helderberg, designated by Conrad 

 as OrtJiis biloba var. varicus, the features of extreme lobation, so 

 ■characteristic of the adolescent and mature conditions of the Orthis 

 ■biloba, are ultimately re-absorbed, and this variety differs at last but 

 little in shape from the non-lobate orthids. The Gothland form known 

 as 0. biloba var. verneuiliamis, Lm., presents a tendency to vary in the 

 opposite direction. The lobation becomes more and more pronounced 

 with increasing age, and the shell exceeds in size the normal species 

 Orthis biloba, which is the directly linear or epacmic form, and enjoys 



