BRITISH SILURIAN BRACHIOPODA. 123 



stretches across the upper part of the spiral cones on their dorsal side, and is in close 

 proximity to them. Each end of the loop first curves upwards, nearly to the top of one 

 of the spiral cones, and then the two parts of the loop are united by a descending V- 

 shaped curve, following the direction of the posterior border of the two spiral cones. The 

 following points may be noticed in the comparison of the spiral arrangement of Zijgospira 

 with that of Atrypa. 



In Zygospira the primary lamellse, shortly after their origin, are bent outwards 

 towards the lateral margins of the shell, more at a right angle than in Atrypa. In the 

 loop of Zygospira, the point of origin from the primary lamellae on either side is lower 

 down than in Alrypa, and thus the loop in Zygospira is broader, and crosses the upper 

 part of the two spiral cones. I have previously observed that in full-grown specimens of 

 Atrypa reticularis, the basis of the spiral cones is quite level, and faces the bottom of the 

 dorsal valve, but that in young specimens of the same species this is not the case, the two 

 inner sides of the principal coils being shghtly higher than the two outer sides, and 

 turned towards the margin of the shell. I observed, also, that in this respect the spirals 

 of A. marginalis and A. Barrandei resemble the spirals in the young specimens of A. 

 reticularis. The same direction of the spiral cones also occurs in Z. modesta, as will be 

 seen in the accompanying figure, only the irregularity between the inner and outer sides 

 of their principal coils is still more marked. The generic difference, therefore, between 

 Zygospira and Atrypa, so far as their spiral arrangement is concerned, must be looked for 

 in the shape and position of the loop, rather than in the direction of the spiral cones. 



Obs. — When describing Atryjm modesta in vol. I, p. 141, 1847, of his ' Palaeontology 

 of New York,' Professor Hall says, " This species, A. recurvirostra, A. dejlexa, and A. 

 exigua form a group presenting characters which may require a separation from the true 

 typical Atrypa.'" 



At page 69 of the 'Thirteenth Annual Report,' 1860, Professor Hall again called 

 attention to the characters of A. modesta, and to the internal spirals of that fossil. At p. 

 154 of the 'Fifteenth Report,' published in 1862, he proposes for such shells as A. 

 modesta a new generic name, Zygospira. " Shell bivalve, equilateral, inequivalve ; surface 

 plicate in the typical species ; a sinus in the dorsal valve. Internal spires arranged some- 

 what as in Atrypa, with a broad loop passing from the outer limbs of the spiral band 

 entirely across from side to side, near to or above the centre, and close to the inner side 

 of the dorsal valve." 



Professor Hall accompanies his description by two figures showing the spirals and 

 loop taken from two different specimens, said to be of the same species, and representing 

 the dorsal and ventral aspects. In his observations on these two very dissimilar speci- 

 mens he adds " The direction of the spires is nearly the same as in Atrypa, differing in 

 the presence of the strong loop ; while the shell, in its interior character, is quite unlike 

 Atrypa. In the two specimens examined there is a slight variation in the direction and 

 extent of the loop. In one it rises from below the middle of the outer curve of the spiral, 



