METAZOA VERMES. 269 



sented in Glottidia and Lingula in PI. 655, figs. 1-4. New 

 tentacles are continually added at the same points, until 

 by pushing back the older ones a complete circle is formed 

 about the mouth (fig. 2), which later becomes introverted 

 in front (fig. 3). From this common and simple struc- 

 ture are developed all the complicated brachia of the more 

 specialized orders (Beecher). In the case of Lingula the 

 growing points at which new tentacles arise, separate and 

 the adult possesses two coiled arms one on each side of 

 the median line (fig. 4). 



We have already seen that members of the Atremata, 

 Trimerella for instance, exhibit specializations of struc- 

 ture illustrating progressive tendencies, but the group 

 contains no old age forms. 



According to Beecher's classification, the Atremata are 

 followed by the Neotremata, Protremata, and Telotre- 

 mata, the first two being the more primitive orders and 

 the last two the more specialized. This arrangement is in 

 accordance with the geological history of the four orders, 

 since the Atremata were the first to appear and the Telo- 

 tremata the last. 



Schuchert's classification 1 gives the following arrange- 

 ment : Atremata, Telotremata, Neotremata, Protremata. 

 This investigator makes a fundamental division of the 

 class into two groups, the Homocaulia or those Brachio- 

 pods in which the pedicle is common to both valves, and 

 the Idiocaulia in which the pedicle is restricted to one 

 valve. The Atremata and Telotremata belong to the first 

 division, according to Schuchert, and the Neotremata and 

 Protremata to the second. 



The fact that the Atremata and Neotremata are the 

 most primitive of the four orders and are consecutive, is 

 not considered a reason by Schuchert for placing the one 

 after the other in a classification. Inasmuch as the line 

 of descent is direct from Atremata to Telotremata and 



1 Bull. U. S. Geol. Surv. , no. 87, 1897, pp. 118-135. 



