540 



Forty- fifth Report on the State Museum. 



triangular, unclosed fissure thus formed, persists throughout early 

 post-embryonic growth-stages. It has already been observed 

 that the period at which delticlial plates begin to appear, and the 

 degree of development which they attain in this group, varies 

 with the species. Thus in most forms of Rhynchosplra and in 

 Meristina Maria they are completed at an early adolescent 

 stage, while in Meristina rectirostra, they are in a primary con- 

 dition of development at maturity, and the pedicle is at no time 

 restricted to a foramen. 



In Magellania when the deltidial plates begin to develop, it 

 has been found that they are secreted by secondary extensions 

 of the ventral mantle from along the margins of the deltidium, 

 these laminar processes beginning at the cardinal margin and 

 meeting though not uniting along an axial line. (Beecheb.) 



It is therefore to be concluded that the deltidial plates are of 

 post-embryonic or secondary growth and that the absence of the 

 body-plate or deltidium in the embryo of Cistella is to be regarded 

 as an instance of acceleration of development in which this 

 primary phase has been omitted. 



There is a difference in the intimate structure of the deltidium 

 and deltidial plates, in punctate shells ; the former being devel- 

 oped from the body of the thoracic lobe and not from the mantle, 

 and has no punctae ; the latter, originating from the mantle, are 

 perforated by its caecal extensions. 



Orbiculoidea minuta, 

 Three growth stages; showing the change from the paterina-stage with transverse cardinal 

 margin, to the circular outline of the adult. Fig. 224 x 36; fig. 225 x 16; fig. 236 x 30; figs. 224 

 and 225 show the form of the protoconch. (Bkkcher.) 



' In many of the brachiopoda the protegulum or embryo shell is 

 retained throughout later growth stages. Tin's is true where the 

 apices of the valves have not become greatly modified by the 

 encroachment of the pedicle. Its valves are subsemicircular and 

 smooth, the hinge-line straight and its general form exemplified 

 in the adult condition of the primordial genus Patkkina. 



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