106 SYNOPSIS OF AMERICAN FOSSIL BRACHIOPODA. [iutu..87. 



opposite sides of a median lobe, or tentacle, gradually separate, aud tlie 

 further ninltiplicatiou of teutacles results in strap-shaped extensions 

 ou each side, which linally assume a coiled form, due to the limited 

 space in which they grow. Therefore the arms iu adult individuals of 

 these genera have a single cirrated edge, extending from their free 

 extremities to the sides of the oral disk, and, continuing posteriorly, 

 unite ou the ventral side of the disk behind the mouth. Each cirrated 

 edge in the adult lophophore apparently has two approximate rows of 

 alternating cirri (Hancock''), bat as they were originally a single row 

 in early stages, this appearance is evidently the result of a crowding of 

 the cirri or a crumpling of the edge. 



Kovalevski ^^ has shown that in Cistella the tentacles also originate 

 in pairs on each side of the dorso-median line, without a central tenta- 

 cle or lobe. The same mode of increase has been shown by the writer^ 

 to be present in Magellan ia and Terebratalia. In young stages of 

 Cistella, Terebratuliua, Magellauia, and other terebratuloid genera, as 

 well as in Thecidea, after the circlet of tentacles is complete the two 

 points at which new ones are added do not separate, but remain close 

 together throughout the life of the animal. In this case the cirrated 

 margin is lengthened by means of lobation and looping, and often by 

 the final growth of a single, median, coiled arm, cirrated on both mar- 

 gins. Gwynia illustrates the comi^leted circle of tentacles about the 

 mouth. Adult Cistella shows an advance in having the anterior mar- 

 gin of the lophophore introverted, making it bilobed. Megathjais 

 is slightly more complicated by two additional lobes. This simple 

 method of increase is further elaborated in the Thecidiidre. In the 

 higher genera, especially among the Terebratulida^, the maximum is 

 reached by means of a median, uni)aired, coiled arm, as in Magellauia 

 aud Terebratuliua. 



The development of the different types and varieties of arm structure 

 is presented in the accompanying figures (figs. 2-G), which are necessar- 

 ily somewhat diagrammatic in order to show the features clearly, but 

 the essential structure can readily be verified from consultation of the 

 works cited or from a study of actual specimens. In the case of fossil 

 forms, such as Dielasma, the Atrypida?, and Athyrida?, the brachial 

 supports have sufficient analogy with the arm structures of Terebratu- 

 liua and Khyuchonella to warrant their interpretation as given. Also 

 the spiral impressions on the valves of Davidsonia, and those occa- 

 sionally present in Leptfena and Productus, clearly point to the posses- 

 sion of coiled arms by these genera. 



CLASSIFICATION OF BRACHIAL STRUCTURES. 



From what has already been shown it is seen that the various types 

 of lophophores admit of a simple classification into stages and groups. 

 It is proposed to give to these distinctive names, which may be used with 

 facility iu making comparisons and correlations. They may be found 



