scHUCHERT.] CLASSIFICATION OF BRACHIAL STRUCTURES. 107 



useful, also, in designating tlie kind of bracliial complexity attained 

 in any genus the arm structure of which can be determined, thus help- 

 ing to fix its place in a genetic scale. It should be emphasized, how- 

 ever, that the form and complexity of the cirrated margin of the lopho- 

 phore can have a taxonomic value only within comparatively narrow 

 limits. This at once becomes evident when the arms of Lingula, Dis- 

 cinisca. Crania, Ehynchonella, and all the Spiriferacea are considered. 

 Each has spiral arms, Avhich were probably developed through similar 

 changes of form, and yet each is genetically distinct, as shown by all 

 the other leading characters. But when this classification of arm 

 structures is applied within a family or genus, or even when made 

 the basis of comparison among some closely related families, it is some- 

 times possible to reach very satisfactory conclusions relating to the 

 systematic position of various forms. 



LEIOLOPHUS STAGE. 



It is hardly necessary to direct attention to the embryonic brachial 

 structure before the growth of any of the tentacles, or cirri, on the edge 

 of the lophophore, while the animal is in the typeinbrj^onic stage. For 

 the sake of designating all the stages, this may be called the leiolophns 

 stage, though it has no special significance beyond indicating the 

 beginning of the lophophore. 



TAXOLOPHUS STAGE. 



The first stage in which a true brachial structure is manifest is an early 

 larval form, often the protegulum stage, when the tentacular portion 

 of the lophophore is a simple arc, or crescent. This may be called the 

 taxoJophiis. The tentacles are few in number, and increase takes place 

 on each side of the median line, dorsally, in front of the mouth. In 

 figs. 2tt, e, 3a, /, 5a this character is clearly shown. The tentacles at the 

 ends of the arc are the oldest, and new ones are being formed in the 

 middle portion. In Thecidea, Cistella, and Magellania the tentacles of 

 the taxolophus are centripetal, due to the edge of the lophoj^hore being 

 near the margin of the shell; while in Terebratulina, Discinisca, and 

 Lingula they are centrifugal, due to the smaller and central lophophore. 



So far as known, there is no adult living form which has the taxolo- 

 phian brachial structure. It may have been present in adult Iphidea 

 of the Cambrian. 



TROOHOLOPHUS STAGE. 



By the continual addition of new cirri and the pushing back of the 

 old ones, the fringed margin of the lophophore passes from a crescentic 

 to a circular form, thus making a complete ring about the mouth. This 

 may be termed the irocJiolophus stage. It appears in the late larval and 

 early adolescent stages of Thecidea (fig. 2?>), Cistella (fig. 2/), Magella- 

 niaand Terebratalia (fig. o/>), Terebratulina (fig. ^g), Glottidia (fig, T)^), 



