420 Proceedings Portland Society Natural History 



shores, the associations of eroding shores having been con- 

 sidered in connection with the xerarch formations (page 

 364). 



a. Association-types of barrier-beach ponds 



The development of barrier-beaches across the month 

 of some reentrant of the shoreline results in the formation of 

 a pond, which ordinarily is salt or brackish, since it is con- 

 stantly fed by seepage through the beach. The vegetation 

 in these salt ponds differs markedly from that of fresh- 

 water ponds, particularly in the aquatic species. Here the 

 most characteristic forms are the eel grass (Zostera mari- 

 na) and the ditch grass (Ruppia maritima), which often 

 occupy the whole area, together with various pond-weeds, 

 such as Potamogeton pectinatus and Potamogeton bupleu- 

 roides. Where the shores are muddy a typical salt marsh 

 is usually present (Figure 47). On rocky or gravelly 

 shores, however, other species are more frequent. On the 

 seaward side, in the areas of greater salinity, grow forms 

 which can tolerate the salt, such as Scirpus americanus, 

 Scirpus campestris var. paladosus, Elcocharis palustris 

 and Spartina Michauxiana. Along the landward side, on 

 the other hand, where seepage from the adjacent uplands 

 renders conditions much fresher are found many species 

 more characteristic of fresh-water areas. These common- 

 ly include : 



Typha latifolia Myrica Gale 



Poa triflora Alnus incana 



Scirpus occidentcdis Polygonum Hydropiper 



J uncus canadensis Spiraea latifolia 



Iris versicolor . Galium Claytoni 



b. Association-types of salt marshes 



The rocky and abrupt character of most of the shores 

 in the Penobscot Bay region naturally precludes the for- 

 mation of salt marshes which are dependent on the deposi- 



