OF CENTRAL CANADA PART IV. 267 



bryozoons. Their chief characteristic is the possession of a pair of 

 long, ciliated " arms/' coiled up within the shell, and supported in 

 some forms by calcined structures. These so-called " arms " undoubt- 

 edly represei t to some extent the tentacular organs of the bryozoons. 

 Living brachiopods are for the greater part attached to the sea-floor 

 by a flexible pedicel, passing through a foramen in the shell or 

 between the bivalves ; but in some cases the attachment is made by 

 the shell itself, without a pedicel. 



In the seas of the Palaeozoic Age, 

 brachipods abounded and far exceeded in 

 numbers the lamellibranchiate mollusca. 

 During the Mesozoic periods, they were 

 still numerous, but they diminished greatly 

 in the succeeding Cainozoic Age, and at 

 present about 100 living species only are FlG - 182> 



r -Di Diagram of a typical brachiopod 



known. Whilst most Of the PaleeOZOlC in normal position. 



genera are extinct, some few as lingula, rhynconella, terebratula, 

 &c. still offer living representatives. 



The two valves of the brachiopod are of unequal size, but always 

 equilateral. As regards the latter character, therefore, a straight 

 line drawn vertically through the middle of each valve will divide 

 the shell into two symmetrically equal parts. This serves to distin- 

 guish at a glance a brachiopod shell from the shells of ordinary 

 bivalve mollusca, or, at least, from the great majority of these, as 

 some few, the Pectens for example, have nearly equilateral shells. 

 The larger valve in brachiopods is almost always the ventral valve ; 

 its " beak " or " umbo " is sometimes very prominent, sometimes 

 depressed or inconspicuous ; and in some genera it is perforated for 

 the passage of the pedicel. The smaller or dorsal valve is in some 

 cases articulated by short projecting processes or " teeth " to the 

 larger valve, whilst in other cases the articulation is very indistinct j 

 and in one group of forms, including the lingulidce, <fec., the valves 

 are without articulation. The two valves along the so-called " hinge- 

 line " (the upper part of the shell as conventionally drawn in figures) 

 are either in close contact or connected by an intervening " area." 

 The latter is usually striated. In its centre, or under the beak or 

 umbo of the larger shell, there is sometimes a smaller triangular 

 area or " deltidium." This is sometimes perforated for the passage 



