106 SYNOPSIS OF AMERICAN FOSSIL 15UACHI0P0DA. [iui,i..87. 



opposite sides of a median lobe, or teutacle, gradually separate, and the 

 further multiplication of tentacles results in strap-shai)ed extensions 

 on each side, which Anally assume a coiled form, due to the limited 

 space in which they grow. Therefore the arms in 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 on the ventral side of the disk behind the mouth. Each (drrated 

 edge in the adult lophophore apparently has two approximate rows of 

 alternating cirri (Hancock-'), but as tlicy 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 Magellania and Terebratalia. In young stages of 

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

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

 points at which new (mes 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 looj)ing, and often bj^ 

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

 gins. Gwynia illustrates the completed circle of tentacles about the 

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

 gin of the lophophore introverted, making it bilobed. Megathyris 

 is slightly more complicated by two additional lobes. This simple 

 method of increase is further elaborated in the Thecidiida'. In the 

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

 reached by means of a median, unpaired, coiled arm, as in Magellania 

 and Terebratuliua. 



The development of the ditferent types and varieties of arm stru(;ture 

 is presented in the accompanying figures (figs. 2-0), 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 Athyridfe, the brachial 

 supports have sufficient analogy with the arm structures of Terebratu- 

 liua and Ehyuchonella to warrant their interpretation as given. Also 

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

 sionally present in Lepta'ua 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 in making comparisons and correlations. They may be found 



