98 SYNOPSIS OF AMERICAN FOSSIL BRACIIIOPODA. IiirLi..87. 



more drawn out beyond tlie dorsal posterior margin than others. If 

 this rostrate condition were carried a little farther and the psendodel- 

 tidium reaorbed, there would practically result atelotrcuiate shell dupli- 

 cated by the neanic condition of many rostrate Telotremata. The 

 articulation would at lirst be nearly obsolete and situated extremely 

 lateral, as in the Protrenuita, but as the cardinal area became greater 

 the teeth would attain a more medial position. While there uro. no 

 known genera to fill in the gap between the theoretical Paterina and 

 Protorhyucha (P. minor and P. amhlgua), yet the hiatus between the 

 Atremata and Telotremata is not greater than between theoretical 

 Paterina and Kutorgina, or between the Atremata and Protremata. 



DEVELOPMENT AND SIGNIFICANCE OF THE DELTIDIUM. 



The most characteristic mature feature of ordinal importance which 

 distinguishes Protremata from the other three orders is found in the 

 plate that more or less completely covers the delthyrium. However, in 

 two of the families of this order, Pentameridte and Orthida', this plate 

 is generally Avauting in the mature individual, since here it usually 

 develops only during early growth, and later is lost by abrasion or 

 hidden beneath the incurved beak. Again, in the Acrotretidie of the 

 Neotremata, and in Iphidea of the Atremata, a deltidium-like plate is 

 also often developed, but as these shells are strongly x)hosphatic it is 

 not difficult to distinguish the ordinal position of any shells with a true 

 deltidium. In Lacazella mediterranean the only living species of Pro- 

 tremata, this plate has its origin in the cephalula stage along with the 

 rudiments of the dorsal and ventral valves, when the embryo is yet free 

 and swimming about by the aid of cilia. The dorsal shell and the pro- 

 deltidium appear first, and are secreted by the rudimentary dorsal 

 mantle and the dorsal surface of the body, which subsequently becomes 

 the pedicle. The ventral shell appears last, and is then widely sepa- 

 rated from the dorsal valve. Between the two valves is the thick and 

 short pedicle, on the dorsal surface of which still remains the third 

 plate, or prodeltidium. Subsequently the latter is anchylosed to the 

 posterior margin of the ventral valve. The prodeltidium is also known 

 in the Atremata and Neotremata, yet in the Telotremata this embryonic 

 third plate does not exist, but a covering to the delthyrium is developed 

 sometime after the animal has become attached. In its origin this cov- 

 ering is wholly different from the deltidium of the Protremata, which has 

 its beginning in the prodeltidium and grows down from the shell apex 

 over the delthyrium, while the deltidial plates of Telotremata grow 

 out medially from the walls of the delthyrium. The deltidial plates are 

 secreted by extensions of the ventral mantle, and at no period of devel- 

 opment has the pedicle any share in their formation. It is not always 

 easy to distinguish mature protrematous and telotrematous shells on 

 the basis of these characters alone, but the young of both orders are 



