382 ROBERT TRACT JACKSON ON THE 



Ehombopteria. Leptodesma orus. Hall, is a species with a well-developed posterior 

 wing but a camera drawing of the young is rhomboidal and bears much resemblance to 

 Rhombopteria. The same comparison may be made in a degree by noting the difier- 

 ences in the younger and older specimens tigured by Hall in several species as L. matli- 

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

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

 logenetic series. 



Avicula. the representative of the family, is clearly derived from Leptodesma-like forms. 

 Using Avicida hiriijido, L., the type species of the genus, as our basis of comparison 

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

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

 developed triangidar cartilage pit exists in all Aviculas. Li species Avith a narrow hinge 

 line, as ^1. sterna, the pit is only prominent in the young; but in others, as in the sub- 

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

 ual we find that A^-icida 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 xmcertain as many eai'ly types called Avicula certainly do not properly belong 

 to that genus 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 Carbonifei-ous. 

 Fi"om Avicula many genera and sub-genera are dei'ived, a part of Avhicli only we are 

 prepared to discuss. (Further consideration of Avicula, see p. 330.) 



Pseudomonotis. The young, as observed in P. ecJiinata. 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 ditferentiated. 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 form and present a high reduction of Aviculoid 

 characters. The}' are unquestionably side-issues off the ascendent line of the grouj) and 

 should probbly be associated with 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 vai-iation fi'om 

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

 are shown in PI. xxvn, fig. 7. (Further dcscnption 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. deciissafa, fig. 35, p. 332. The hinge 

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



'A cartilage pit is eharacteristic of the Pecten division teria and Leptodesma. In all it may be inherited from the 



of this group and it is probably characteristic of Khombop- Xuculoid ancestor in wliich it is a characteristic feature. 



