0. E. Beecher — Development of the Brachiopoda, 347 



No marked variation lias yet been observed among the spire 

 bearing genera, nor has any been seen in the terebratuloids or 

 rhynchonelloids further than the radii on the protegulum 

 of Atretia (Cryptopora). Possibly this feature in Atretia is an 

 inheritance from the radiate character of the shell in the 

 Phynchonellidge. It may be, however, one of the features 

 consequent upon its fragile nature and deep sea habitat, as 

 observed among other abyssal shells. 



Differences in the Valves. 



The dissimilarity in the form and relations of the two valves 

 progressively increases in the following genera: Lingula, 

 Terebratulina, Cistella, Discinisca, Thecidium (Lacazella), and 

 Crania. Lingula is nearly equivalve, both valves bearing a 

 close resemblance to each other. In Terebratulina and Cistella, 

 the two valves are more strongly specialized, while in Dis- 

 cinisca, Thecidium, and Crania, they are quite unlike. 



Two important organic characters accompany and partake of 

 a similar amount of variation ; (a) the length and direction of 

 the pedicle, and (h) the position and structure of the pedicle 

 opening. Lingula with a long, fleshy, mobile pedicle receives 

 uniformly disposed axial impacts on the valves, and, therefore, 

 with equal physiological reactions, equality in size and form is 

 produced. Terebratulina and most of the other terebratuloids 

 and rhynchonelloids have a shorter and less flexible pedicle. 

 As a whole the motions of the animal are more restricted ; the 

 pedicle opening is confined mainly to one valve ; the valves, 

 consequently, are differently related to the environment, and 

 express this difference in their dissimilarity. In these exam- 

 ples, also, the inclination of the pedicle to the longitudinal 

 axis, or of the shell to the surface of support, agrees, pari 

 passu with the amount of unlikeness in the valves, except 

 when the pedicle is so shortened as to interfere with their 

 free movement. To this inclination is probably due the 

 difference in the action of the forces from without. 



Normally, in Lingula, the pedicle is in direct linear contin- 

 uation with the axis of the shell. Terebratulina and Magel- 

 lania are inclined at an angle of 40° to the surface of support, 

 but in Cistella and Muhlfeldtia, this is increased to about 70°. 

 In the latter genera, although the position of the axis is nearly 

 vertical, the shortening of the pedicle precludes more than a 

 slight elevation and rotation of the organism. The more the 

 pedicle opening is confined to one valve the greater is the 

 difference between both. 



Passing to Discinisca, the pedicle is found to be at right 

 angles to the longitudinal axis, and the valves become 

 strictly an upper and a lower. The lower rests upon the 



