378 C. E. Beecher — Hevision of the Families of 



3facandrevia, composed of two descending and ascending lamellae, 

 was believed to be homologous with the loop of Terehratulina and 

 Liothyrina^ and the family proposed by King fell into disuse. It 

 can now be shown, however, that the loop of 2Iacandrevia is made 

 up of a primary portion corresponding to the entire loop of Liothy- 

 rina, and a secondary part which has no equivalent in the calcified 

 lamellae of Liothyrina nor Terehratulina, but in them is represented 

 in thf^ fleshy portion of the arms, as previously recognized by 

 Hancock.^^ 



The loop in Terehratulina is equivalent to the descending lamellae 

 in Terehratella, from the crural points down to and including the 

 bands connecting with the septum. In Magellania and Macan- 

 drevia, the connecting bands of Terehratella are represented, except 

 in old specimens, by slight projections from the descending 

 branches, and in these genera, therefore, the primary loop is incom- 

 plete.* The true relations and homologies of these parts can best 

 be shown in a series of figures. 



Plate I, figures Cl, Dl, represent the loop in a young Macan- 

 drevia cranium m the so-called platidiform stage, showing a com- 

 l^lete primary loop and the beginning of a secondary loop in the 

 middle, on top of the septum. A later stage of the same species, 

 Plate I, figure Gl, has the structure of Terehratalia. The descend- 

 ing lamellae and the median V-shaped plate correspond to the pri- 

 mary loop, while the secondary loop or posteriorly recurved portion 

 has greatly increased in size. A later stage, nearh^ complete, Plate 

 II, figure 1, shows two points (p) on the descending lamellae, which 

 are remnants of the connecting band in previous stages. The parts 

 homologous with the loop of the first stage, and with the loop of 

 Terehratulina , are shaded. Greater emphasis is expressed by figures 

 2, 3, Plate II, where the cirrated brachia and calcareous supports 

 are both represented in the genera Terehratulina and Magellania. 

 It is readily seen that the arm structure is the same in both, but 

 that the calcareous loops which are darkly shaded are very different 

 in form. 



The family Terebratellidae should therefore be reinstated on the 

 evidence here given. The development of Terehratella may be 

 reviewed for the leading characteristics of one division of the family. 



* The prongs, or points, below the ends of the crura on the primary lamellae in 

 Spirifer also represent portions of a loop. More close analogy is seen in later forma 

 of Atrypa having a disunited loop. 



