DISTEIBUTION. 315 



The pedicle tube of Keyserlingia (PI. LXXXI, fig. 4c) apparently opens into an elongated 

 depression in the shell back of the apex of the ventral valve ; the tube is a passage through 

 a tliickened mass of shell which is dejiosited about the pedicle and wliich is especially well 

 developed in old shells (PL LXXXI, figs. 4a and 4d). 



PROTBEMATA. 



A pedicle tube or opening is found in the genus BiUingseUa of the Lower Cambrian, the 

 characteristic openiiig being at the apex of the delthyrium (fig. 65, p. 750). It has not been 

 observed in Nisusia (PI. C) nor in the subgenus Jamesella (PI. CI), nor in any other genus or 

 subgenus of the Protremata of the Cambrian fauna. This may be owing to the absence of a 

 deltidium in the adult stages of the shell found, to its being covered by the incurved beak, to 

 the difficulty of worldng out or uncovering the deltidium so that it can be seen, or in all proba- 

 bility to the permineralization of the shell and the generally coarse nature of the sediments 

 deposited with the shells. It may be that the opening was closed naturally in the adult stages 

 as in some of the Ordovician Clitambonites. Even in BiUingseUa, where the material is abundant 

 and well ])reserved, it is qnly rareh^ that an entire deltidium is obtained and very rarely that 

 the minute pedicle opening can be seen at its apex. 



OLD-AGE CHARACTERS. 



I have not given special attention to old-age characters during the progress of the study 

 of the Cambrian brachiopods. The variations due to old age during the gerontic period appear 

 in the unusual thickening of the shell of Oholus apollinis (PL VII), but old age is not indicated 

 on the exterior of the shell (PL XIV, figs. 6 and 6a). The areas of attachment of the muscles in 

 Oholus mickwitzi (PL X) suggest old age, and the strong concentric ridges at the front of the 

 valves is another phase of old-age conditions. Oholvs {Lingulobolus) (PL XVI), in its thick 

 shell, large deep muscle scars, and convex valves indicates an old-age form derived from Oholus. 

 In Keyserlingia huchi (PL LXXXI, figs. 4a-e) the interior shell growth associates the species 

 with old-age forms. 



There is abundant material for study of many of the species described in this volume that 

 may, when carefully worked up and studied, give some interesting results in relation to changes 

 in the shell during the mature (neologic) and old-age (geratologic) periods of growth. 



DISTEliBUTION IN CAMBRIAN STRATA. 



We do not know of any brachiopods in strata older than that containing the Mesonacidse 

 or Lower Cambrian fauna. That such existed in pre-Cambrian time seems almost certain when 

 the advanced stage of development of some of the earliest-known forms is considered. 



In the following diagram the Icnown occurrence of the families of bracliiopods in strata of 

 Cambrian age is graphically shoA\ai. The diagram is based on the data contained in the table 

 (p. 112) giving a summary by famihes. The Obofidw, with 6 genera, 9 subgenera, 186 species, and 

 14 varieties, has the greatest development, and tliis family continues into the base of the Ordo- 

 vician with 4 genera, 3 subgenera, 41 species, and 1 variety. The Acrotretidse has 6 genera, 



1 subgenus, 100 species, and 20 varieties, with the greatest development in the Mddle Cambrian 

 and with a smaller representation m the Lower Ordovician. The BillingseUidas, with 9 genera, 



2 subgenera, 91 species, and 12 varieties, has a strong line of development from the upper 

 Middle Cambrian and passes into the Ordovician, where it disappears. The three families 

 mentioned include about 48 per cent of the genera, 80 per cent of the subgenera, 80 per cent 

 of the species, and 79 per cent of the varieties included in the Cambrian Brachiopoda. The 

 development of genera of the remainmg families containing .3 genera or more is as follows: 

 Paterinidse, 3; OboleUidaj, 4; Siphonotretidse, 6; Syiitrophiidaj, 3; or 16 genera of the 23 outside 



