106 



DE, T. DAVIDSON ON EECENT BEACHIOPODA. 



Eia 



denticulated calcareous plates, and vary suflBciently in arrangement in different genera of 

 Brachiopoda to serve as distinguishing characteristics (Plate XIX. figs. 18 & 19). 



M. E. Deslongchamps describes with much care and detail, at page 117 of his valuable 

 memoir ' Notes sur les Modifications a apporter a la Classification des Terebratulidte,' 

 1884, the different stages in the development of Megerlia truncata. He says that 

 " the modifications the shell undergoes from the embryonic to the adult condition present 

 a special character, and constitute a rather peculiar type, at least so far as concerns the 

 brachial appendages. The shell assumes from the commencement a shape which is 

 but little modified by age. It is only more flattened and nearly circular in the youngest 

 individuals; ribs obscurely nodulous then mark the surface of the two valves. The 

 foramen is nearly triangular, but with an elliptic shape. In growing, the shell becomes 

 more and more transverse and sub quadrilateral, with a slight tendency to indentation in 

 front. The foramen enlarges posteriorly, but is never provided with more than two 

 rudimentary deltidial plates even in its most adult condition. 



" Rather curious and much more accentuated changes take place in the brachial 



appendages. We will examine them first in a shell two millimetres in length The 



hinge-plate is very largely developed .... two small 

 projections, c, indicate the places whence the principal 

 stems of the loop will afterwards originate, but of which 

 there is as yet no indication. Towards the middle of 

 the bottom of the dorsal valve rises an apparatus 

 very different from that we have hitherto ob- 

 served, which first develops round the mouth. 

 Two smaU pillars, P, P, rise from the bottom 

 of the valve, then expand forward, forming two 

 thin lamellae, and expand in the shape of two half- 

 moons, I, I', furnished inside with small irregular 

 asperities, and miited at their two inner extremities, 

 leaving free a little triangular space, t. This small 

 space, to which we wiU apply the name ' escutcheon,' 

 is completed above by the borders of the semilunar 

 expansions just mentioned, and there constitutes tlie 

 deflected lamellae. These are united by a lamella in 

 the shape of a bridge, P*, which frames the escut- 

 cheon. The portion of the loop therefore that is first 

 formed is that in the region of the deflected ajiophysis, 

 and then no trace of the principal stems of the loop 



■ P. I.e. 



Young example of Majerlia truncata, 2 

 millimotres iu length (enlarged), after 

 M. E. Deslongchamps. 



P.l.c, hinge-plate ; c, c, Ist indications of 

 the principal stems of loops forming two 

 smaU isolated projections ; P, P, small 

 pillars rising from the bottom of the 

 valve, expanding forward in the shape 

 of two half-moons, Z, l\ furnished with 

 points ; t, the escutcheon ; P*, united 

 lamellae in the shape of a bridge. 



exists. This is contrary to what takes place in aU 

 the Brachiopoda we have hitherto recognized. (See woodcut, figure 9.) 



" At the dimensions of 4 millimetres these two semilunar expansions, I, I', have become 

 elongated, and while becoming thinner have lost the small accessory projections of the 

 first stage. The Y-shaped apophysis that connects the deflected lamellae at the bottom of 

 the valve forms a process more or less analogous to the pillar of Platydia ; but their 



