28 INTRODUCTION. 



all of which are drawn under the same magnifying power, namely 100 diameters. Among 

 the recent Terebratulae, there is none which have canals of larger size or more closely set 

 than Waldheimia australis (PI. IV, figs. 6, 7, 8), their average diameter being about 

 jigth of an inch, and the distance of their centres about ^^th. It is in TerebratuUna 

 caput-serpentis (PI. IV, figs. 11, 12), on the other hand, that the canals are of smallest 

 dimensions, their largest diameter being about ^^th of an inch, whilst their average 

 distance from each other is about the same as in the preceding case, — their regular 

 arrangement, however, being so modified, that the external orifices are principally seen 

 upon the elevated parts of the plications (fig. 12), whilst they open internally in similar 

 rows (fig. 11). Among the fossil species of Terebratulae, Ter. bullata (PI. V, fig. 1) is among 

 the most remarkable for the size of its canals, their diameter being about 5^th of an inch ; 

 whilst in Ter. lima (PI. V, fig. 2), whose shell-canals are smaller than those of any 

 Terebratula, recent or fossil, that I have examined, their diameter is scarcely ^on)-ot^ of 

 an inch. The relative distances of the centres of the canals, however, are nearly the same in 

 these two species, averaging about ^-q th of an inch, so that about the same number may be 

 counted in any given area ; thus showing that there is no fixed relation in the development 

 of the canal-system, between their size and their number. The largest canals which I 

 have met with in any Brachiopod shell, are those of Sjpiriferina rostrata (PI. V, fig. 3), 

 these being about ^pfo-tli of ^^ inch in diameter. Their number in a given area, however, 

 is much less than it is in the Terebratulidae generally, the distances of their centres 

 averaging about y-B-o ^^ of an inch ; so that the proportion which their united arese bear to 

 that of the shell is not very different from that which obtains in Terebratula australis, 

 or Ter. bullata, — the diameter of each canal being, in these species, from about one-half to 

 five-eighths of the distance between the centres of the contiguous canals. 



In all these statements, however, the lar(jest diameter of the canals has been alone 

 considered ; and this, in the TerebratuUdce, is usually near the exterior of the shell, the 

 canals expanding as they approach this, so as usually to present somewhat of a trumpet- 

 form (PL IV, figs. 3, 4). Hence, the diameter of the internal orifices of the canals is 

 frequently not more than one-half, sometimes not more than one-third, of that of the 

 exterior ; and it is still further reduced, when a new layer of shell has been formed within 

 the preceding (as already noticed, p. 24). Of this, a good illustration is shown in PI. IV, 

 fig, 3, where d d marks the plane of junction between the new layer and the old (its 

 position in the shell being seen at d d, fig. 1), below which the canals are seen to be con. 

 tracted to about one-half the diameter of their ordinary internal terminations. Sometimes 

 the canals are seen to bifurcate in their passage from the interior towards the exterior of 

 the shell (PI. IV, fig. 4) ; this bifurcation, however, is not common in Terebratulidae. The 

 external termination of each canal is covered with a sort of discoidal operculum (PI. IV, fig. 8), 

 which is much thicker and more opaque than the periostracal membrane ; to this it usually 

 remains adherent when the calcareous shell has been dissolved away by dilute acid, but 

 it is occasionally found to be detached from it, so that the adhesion can scarcely be 



