no. 1G46. REVISION OF BEYRICHIID.E—ULRIC II AND BASSLER. 281 



(angular line drawn from the middle of the straight cardinal edge 

 divides the area of the valve into two more or less unequal parts of 

 which the posterior is the longer and usually the greater. In other 

 words, the valve is more or less oblique and its outline suggests a 

 parallelogram rather than an oblong. Now, in by far the majority 

 of Primitiidse and Beyrichiidse, the narrower and, rather less gen 

 erally, the thinner half of the carapace is determined to be anterior 

 also by the retral swing of the outline and the comparative analysis 

 of the nodes, lobes, and furrows. 



The retral (parallelogram) swing of the outline is perhaps the 

 most persistent of the criteria, being applicable in even those cases 

 (certain Leperditellida 1 ) in which the valves are without nodes, their 

 surface uniformly convex, and the ends nearly or quite equal in height. 

 In many of the true Beyrichise the resulting obliquity of outline and 

 inequality of the ends are both very inconspicuous (as, for instance. 

 B. kochii, B. maccoyiana, B. salteriana), and in this genus it is often 

 necessary in deciding which is the right and which the left valve to 

 rely almost entirely on the correlation of the lobes. The data for 

 this correlation are furnished by species like Beyrichia clavata, in 

 which the " swing " and the difference in height of the two ends is 

 sufficient to leave no doubt as to which is the anterior. A study of 

 such a species shows that the median lobe is united below with the 

 larger anterior lobe by means of a low and thin isthmus, and that the 

 posterior lobe, if its ventral extremity extends forward at all, passes 

 beneath this isthmus. It is observed further that the median lobe is 

 located nearer the posterior than the anterior lobe; in other words, 

 that the anterior furrow is almost without exception the wider of the 

 two. Now, 1 tearing these facts in mind, the anterior lobe is recog- 

 nized at once as the one that is connected below with the median 

 lobe. When this ventral union of the anterior and median lobes is 

 obsolete, as in B. tuberculata and its immediate allies, the posterior 

 lobe is usually recognized by the location of the median lobe which, 

 as said, is commonly placed more or less distinctly behind the center 

 of the valve. When this and all other tests seem indecisive, as they 

 may rarely be in a species like B. bronni Renter, then it is still pos- 

 sible to orient the valves by comparing minor nodes and furrows on 

 the lobes with similar markings on less difficult species. 



Additional evidence tending to show that the criteria relied on by 

 Lhe writers in orienting the valves of Beyrichiidse is furnished by the 

 Chazy ostracod erroneously referred to Beyrichia by Jones under the 

 lame B. clavigera. This species, though strikingly like a Beyrichia 

 n having a median node within the bend of a strongly curved, low 

 •idge, seems yet to belong to the Leperditiidse. Tt has an eye tubercle 

 md agrees in all other respects, save the curved ridge, with species of 



