ORDOVICIAN BRACHIOPODA in 



3ala costella. This distinction is not considered important enough to merit system- 

 atic recognition so that the species may be regarded as a noteworthy homogeneous, 

 short-ranging stock especially characteristic of the Lower Llanvirn of Britain. 



The Llandeilo species T. immatura from the Meadowtown Beds is more closely 

 related to T. prototypa than was originally thought when the two species were 

 erected. Both species are similar in dorsal outline and the relative sizes of internal 

 features. They differ however in a number of important characters. In T. immatura 

 the ribbing is significantly coarser (p > o-ooi) with a suggestion that externally 

 branching ribs are more commonly developed in Sector IV ; the growth in relative 

 depth of the pedicle valve is faster (p < o-ooi) ; and the lateral spread of the brachio- 

 phore bases is significantly greater (0-05 > p > 0-02). This last difference reflects 

 not only the more divergent disposition of the brachiophore bases in T. immatura 

 but also their excessive thickening by secondary shell accretion in mature stages of 

 growth. 



A small sample of Tissintia from the Betton Beds of Shelve, although strati- 

 graphically intermediate between the two species, was identical with T. prototypa 

 in the key differences. Rib counts per mm, 5 mm antero-medially of the umbones 

 of 2, 3 and 5 brachial valves were 4, 5 and 6 ; the basic statistics for length (1) and 

 depth (th) of 11 pedicle valves were: I (var 1) = 7-68 (3-872), th (var th) = 1-53 

 (0-27) and r = 0-909 ; and for the length (1) and maximum lateral extension (w) 

 of the brachiophore bases in 11 brachial valves they were : I (var 1) = 1-15 (0-137), 

 w (var w) = I'll (0-179) an d r = 0-936. Allometry prevailed in both the deepening 

 of the pedicle valve and the extension of the brachiophores, but neither growth rates 

 nor shape parameters differed significantly from those of T. prototypa. 



The only other species assigned to the genus is T. convergens from the Llandeilo 

 of Morocco (Havlicek 1971 : 51), which appears to differ from the British species, 

 at least, in being relatively more transverse and in possessing smaller brachiophores 

 and longer, more flabellate ventral muscle scars. 



Table 77 



Statistics of length (1) and length of brachiophore bases (lc) in n brachial valves of Tissintia 



prototypa (Williams) from the Weston Beds (A) and the Lower Llanvirn of Llandeilo (B), and of 



T. immatura (Williams) from the Meadowtown Beds (C) 



ABC 



n 21 55 64 

 1 mm (var 1) 8-87 (14-326) 8-59 (4-4) 6-03 (14-756) 



rcmm(varlc) 1-4(0-303) 1-17(0-054) 1-05(0-361) 



r 0-941 0-871 0-966 



log e l (var log e l) 2-0986 (0-1674) 2-122 (0-0578) 1-6267 (°'34°4) 



logelc (var logelc) 0-2685(0-1426) 0-1384(0-0387) —0-0903(0-2824) 



r e 0-944 °'875 0-972 



a (var a) 0-9229 (0-00485) 0-8182 (0-00295) 0-9108 (0-00075) 



