350 TEREBRATULID^. 



their origin from each side of the beak of the imperforate 

 and inferior valve. 



Animal with strongly cirrhated, looped, or contorted 

 arms fixed to the apophysary skeleton. 



Among living Brachiopods the species of the genus 

 Terebratula, restricted as we here define it, may at once 

 be recognised by their punctated shells with perforated 

 beaks and foramina, in most instances completed by a 

 deltidium, whilst on opening living or well-preserved dead 

 specimens, the skeleton will be seen presenting the form 

 of an apparatus composed of shelly or horny loops. The 

 majority of living species of the old genus Terebratula, and 

 an immense number of fossil forms ranging even to the 

 Palaeozoic epoch, belong to the genus so defined. 



M. Alcide d'Orbigny proposes a minuter subdivision of 

 the group, going so far as to regard the genus, even as 

 above restricted, in the light of forming part of two 

 families. In one group he includes four genera, viz., Tere- 

 bratula, distinguished by having no area and a round 

 foramen, encroaching more upon the beak than on the 

 deltidium, which is composed of two pieces ; Terebratella, 

 having an area, and a two-pieced deltidium, upon which 

 the foramen encroaches ; Terebrirostra, having a delti- 

 dium formed of a single piece, and a foramen encroaching 

 upon it ; and Fissirostra, having a similar deltidium, but 

 which is not encroached upon by the foramen ; the latter 

 being confined to the beak, and placed on its outer portion. 

 The only one of these genera which includes an existing 

 British species is Terebratula, in which T. cranium has its 

 place. Our other native form, the caput serpentis, is placed 

 by M. d'Orbigny in his Terebratulina, forming part of his 

 family Magasid<c, in which the shells have no deltidia ; 

 and he distinguishes the genus from Mayas by its trim- 



