382 ROBERT TRACY JACKSON ON THE 



Rhomboi^teria. Leptodesma orus, Hall, is a species with a well-developed posterior 

 wing but a camera di-awing of the young is rhomboidal and bears much resemblance to 

 Ehombopteria. The same comparison may be made in a degree by noting the difler- 

 ences in the younger and older specimens figured by Hall in several species as L. math- 

 eri and L.jason and it may also be observed by tbllowing the lines of growth in many 

 of his figures. Leptodesma is therefore considered the next step in the ascendent phy- 

 logenetic scries. 



Avicula, the representative of the family, is clearly deriA^ed from Leptodesma-like forms. 

 Using Avicvla Mrundo, L., the type species of the genus, as our basis of comparison 

 we find that Avicula differs from Leptodesma in having a highly produced posterior wing, 

 a deeji byssal notch in the right valve, and small cardinal teeth. A more or less well- 

 developed triangular cartilage pit exists in all Aviculas. Li species with a narrow hinge 

 line, as A. sterna, the pit is only prominent in the young; but in others, as in the sub- 

 genus Meleagrina, it is a marked feature of the adults.^ In the ontogeny of the individ- 

 ual we find that Avicula at the prodissoconch stage is referable in ancestry to the Nu- 

 culoid radical of the group. In the early nepionic stage, fig. 32, p. 330, it is referable 

 to Rhombopteria ; in the later nepionic stage, fig. 33, p. 330, a considerable wing has de- 

 veloped and the form approaches near to the winged Leptodesmas. Later still the full 

 Avicula features are acquired. Thus the ontogeny of the individual represents very fully 

 the phylogeny of the genus. The first appearance of true Avicula in the geological 

 horizon is uncertain as many early types called Avicula certainly do not properly belong 

 to that geniis and the limitations of the genus in the past have not been fully ascer- 

 tained. It probably is truly i-epresented as early as the Devonian or Carboniferous. 

 From Avicula many genera and sub-genera are derived, a part of which only we are 

 prepared to discuss. (Fui'ther consideration of Avicula, see p. 330.) 



Pseudomonotis. The young, as observed in P. ecldnata, has a prodissoconch and nepi- 

 onic stage like the same period of Avicula, figs. 32-34, p. 330. Later, the hinge line 

 becomes relatively shorter and very moderate abbreviated wings are difterentiated. Mo- 

 notis occurs later in time and has features closely comparable to Pseudomonotis, there- 

 fore it is considered provisionally as derived from that genus. Daonella and Halobia 

 are genera which are sub-rhomboidal in foi-m and present a high reduction of Aviculoid 

 characters. They are unquestional^l}' side-issues off the ascendent line of the grou]) and 

 should proljbly be associated Avith Monotis or Pseudomonotis, but as I have not had speci- 

 mens for study they are not further considered. (Further description of Monotis and 

 Pseudomonotis, see pp. 331, 332.) 



Oxytoma, a genus near to Avicula, has a short hinge line with moderately developed 

 ears and a slight obliquity of the body of the shell. It seems to be a variation from 

 Avicula, in the same line as Monotis and Pseudomonotis. Some featiires of the young- 

 are shown in PI. xxvii; fig. 7. (Fui-ther descri|)ti()n of Oxytoma, see p. 332.) 



Cassianella is highly inequivalvular; the young has a prodissoconch and nepionic 

 stages like the young of Avicula, as shown in C decussata, fig. 35, p. 332. The hinge 

 line is of moderate length, the wings are moderately produced and cardinal teeth are 



'A cartilage pit is fliaractt'i-i.stic of tlie Tocton division toria ami lA'ptoticsma. In all it may be inhcTitcd from the 



of this groui) and it is probably eliaracteiistic of Hhondjop- Nuciiloid ancestor in which it is a characteristic featnre. 



