BRITISH SILURIAN BRACHIOPODA. 



135 



Further researches will be necessary before it can be possible to offer a definite 

 classification of the spiral-bearing Brachiopoda in their respective groups. The 

 following provisional sketch, however, has been drawn up by Mr. Glass and myself in 

 order to assist those Palaeontologists who may feel nclined to continue the investigation. 



Families or Groups 

 of spiral" 

 Brachiopoda 



1. SpiriferidcB, 

 King, 1846 

 (but much 

 restricted) 



Spirifera 



Cyrtia 



Syringothyris 



Cyrtina 



Ambocoelia 



Uncites 



Spiriferina 



Suessia 



Reticularia 



2. NucleospiridcB. 

 Dav., 1881 



Nucleospira 



Retsia 



Meristina 

 ? Trematospira 



3. Athyridce, 



Phillips, 



1841 

 Aihyris 

 Kayseria 

 Whitjieldia 

 Bifida 

 Merista 

 Meristella 



4. Atrypidce, 



Dall, 1877 

 Atrypa 

 Coslospira 

 Thecospira 

 Anoplotheca 

 Koninckina 

 Koninckella 

 Davidsonia 

 Glassia 

 Zygospira 



Direction o£ spirals. 



The bases of the spirals 

 facing each other in the 

 centre of the shell, but 

 usually with their apices 

 having a more or less 

 upward direction to- 

 wards the posterior 

 angle of the lateral 

 margins of the shell 



The spirals are some- 

 times directed back- 

 wards into the rostral 

 cavity of the ventral 

 valve, as in Cyrtina 

 heteroclita 



The bases of the spirals 

 facing each other in 

 the centre of the shell 

 the apices of the spirali 

 being directed towards 

 the lateral margins of 

 the shell 



A.S in Nudeospiridce 



Straight 



The bases of the spirals 

 directly facing the 

 bottom of the ventral 

 valve, or more or less 

 turned outwards to 

 wards the lateral mar- 

 gins of the shell 



In Thecospira the apices 

 of the spirals face th 

 bottom of the ventral 

 valve 



Attachments 



to 

 hinge-plate. 



The primary la- 

 mellae shortly 

 after attachment 

 to hinge-plate 

 bent backwards 

 towards ventral 

 valve 



As in Nudeospi- 

 ridce 



The primary la 

 mellse shortly 

 after attachment 

 to hinge-plate 

 bent outward 

 towards lateral 

 margins of shell 



Shape of loop. 



Either, as in Spirifera o 

 Cyrtia, consisting of 

 two processes directed 

 downwards but not 

 uniting 



Or, as in Cyrtina, 

 two processes directed 

 downwards but uniting 

 at an acute angle 



Or, as in Spiriferina and 

 Uncites, a straight or 

 curved horizontal band. 

 In Spiriferina this band 

 is simple ; in Uncites 

 it has a slight upward 

 projection in the centre 



Simple, V-shaped, with 

 sharp angular 

 tremity, sometimes 



prolonged into a spine 

 like process 



A more complex loop 

 than in Nucleospira, bi 

 furcating, and variously 

 extended after the two 

 primary lamellae of th 

 loop have become 

 united 



towards 

 side, as 

 Cyrtia. 



Either between the 

 spirals and directed 

 backwards 

 their ventral 

 in Spirifera, 

 and Cyrtina. 



Or between the spirals. 

 but almost on a level 

 with their dorsal sur- 

 face, as in Spiriferina 

 and Uncites. 



Simple, uniting by 

 downward V-shaped 

 curve, but not having 

 a sharp extremity 

 (Whitfield has figured 

 the loop of some exam 

 pies of Atrypa reticu 

 laris as having a sharp 

 extremity. Neither 



Mr. Glass nor myself, 

 however, have met with 

 such examples.) 



Position of loop 



in relation 



to the spirals. 



Between the spirals and 

 near their centre. Di- 

 rected almost horizon- 

 tally from dorsal to 

 ventral side of spirals 



Between the spirals, and 

 extending upwards 

 from about their centre, 



Above and exterior 

 the spirals. 



19 



