78 STUDIES IN EVOLUTION 



to be observed in other living species of Bracliiopoda (CVs- 

 tella, some Dallina, etc.)- Some of tlie extinct genera show 

 more higlily developed cardinal extremities which are often 

 very characteristic of certain species, though considerable 

 variation is found to exist. It is evident that these elon- 

 gated hinge-lines have arisen from the mechanical necessities 

 of a functional hinge, and their greater or less extent is also 

 to a degree dependent upon the nature of the object of sup- 

 port, which furnishes a stimulus to the growing ends of the 

 hinge. A marked example is shown in Spirifer mucronatus, 

 60 with the cardinal angles extended 



into spiniform processes (figure 60). 



Similar features are presented by many 

 Figure 60. - Dorsal o^l^^r species of Spirifer, Orthis, Lep. 



view of Spirifer mucrona- tcena, Stropheodonta, CtC. 



tus; Devonian; showing jj^ ^-j^g Trilobita the pvgidium, or 



spiniform cardinal angles. , t . i . . » 



X |. (After Hall and abdominal portion, consists of a num- 

 Clarke.) l^er of Consolidated segments, and the 



segments of the thorax are successively added in front of this 

 tail piece. The first thoracic segment is therefore formed 

 between the cephalon and pygidium, and its form is mechani- 

 cally in agreement with the requirements of the animal for 

 bending the body, and with the adjacent margins of the 

 cephalon and pygidium. In a way it may be said that the 

 segment is moulded by the adjacent parts, and may there- 

 fore take its form from the cephalon (figure 61), or from the 

 pygidium, as in the examples following (figures 62-65). 



During growth the new segments are added in front of the 

 anal segment, so that after the number of abdominal seg- 

 ments is complete the thorax is increased by the successive 

 addition of what in earlier moults were pygidial segments. 

 By this means the pygidium generally controls or determines 

 the character of the segments of the thorax. If the pleura 

 of the pygidium are extended into spiniform processes, the 

 pleural ends of the segments are also spiniform ; as in Lichas 

 (figure 64), Ceraurus, Cheirurus (figure 62), Deiphon (figure 

 63), Acidaspis, Dindymene, etc. 



