Article: The growth and shell microstructure of the thecideacean brachiopod Moorellina granulosa (Moore) from the Middle Jurassic of England
P. G. Baker
Analysis of the growth habit of M. granulosa from a functional point of view has proved to be of value in the interpretation of shell microstructure. Serial sectioning of shells at 20 p intervals has revealed that fibre orientation may change suddenly at various levels within a shell. The paper notes the need for detailed information regarding the orientation and location of sections through shells, as study of M. granulosa indicates this may be of critical importance. The shell-structure differs markedly from that of Lacazella mediterranea (Risso), as the shells of some, if not all Inferior Oolite thecidellinids were differentiated into primary and secondary layers. Interpretation of the microstructure has taken into account the effects of shell resorption in the brachial valve and the development of crescentic tubercles in the pedicle valve. Some evidence has been obtained which indicates that the pedicle opening of M. granulosa occupied a supra-apical position. Despite the general spiriferoid appearance of the shell-structure, the detailed microstructure of various morphological features of the two valves, together with bulk morphological similarities, are thought to suggest strophomenoid affinity.